A list of mods for book collectors.

BOOK FIXES

http://www.nexusmods.com/skyrim/mods/19/?, http://www.nexusmods.com/skyrim/mods/23491/?, http://www.nexusmods.com/skyrim/mods/25127/? and http://www.nexusmods.com/skyrim/mods/31083/? have numerous fixes for books. For example, the Unofficial Dragonborn Patch adds missing object bound sizes for several books added by the DLC.

BOOK SHELF SCRIPTS

http://www.nexusmods.com/skyrim/mods/10891/? - Uses a script which "places books based on their width rather than fixed positions." The result is that you can place more books on a shelf and they will also generally end up better placed than without it. As a bonus you can also place any other item on the shelf, not just books. Some have reported issues with the mod and it needs cleaning with TES5Edit, but it has always worked perfectly fine for me.
http://www.nexusmods.com/skyrim/mods/24359/? - Same function as Unlimited Bookshelves, but it uses and thus requires SKSE. It "allows you to place up to 48 books in one shelf (based on books size)." Seems to work fine for most users, but I have issues with: it takes a long time for books to be placed and removed and they also often end up jumbled up in a mess. Might be because I'm using several script heavy mods perhaps.

BOOK COVERS

http://www.nexusmods.com/skyrim/mods/35399/? - A "high quality retexture of all the readable books, journals, and notes in vanilla Skyrim and all three DLC. It gives each book and journal its own unique cover and adds many additional paper styles to the notes." See also http://www.nexusmods.com/skyrim/mods/50441/?, where you'll find patches for SkyRe, Requiem and other mods.

http://www.nexusmods.com/skyrim/mods/57120/? - Which "brings high quality covers in the style of Book Covers Skyrim to a very broad range of over two hundred books from previous Elder Scrolls games and their expansions. It is a logical extension to Book Covers Skyrim and is fully compatible with that mod."

ADDED BOOKS

http://www.nexusmods.com/skyrim/mods/57120/? - Which "brings high quality covers in the style of Book Covers Skyrim to a very broad range of over two hundred books from previous Elder Scrolls games and their expansions. It is a logical extension to Book Covers Skyrim and is fully compatible with that mod."

http://www.nexusmods.com/skyrim/mods/5398/? - Which "contains almost 200 books from previous Elder Scrolls games including Daggerfall, Morrowind (including all 36 of Vivec's sermons), and Oblivion! Also included are some never-published works written by Elder Scrolls developers. It also increases the number of book vendors and is compatible with the Unlimited Bookshelves mod!"

http://www.nexusmods.com/skyrim/mods/10462/? - "Tamriel Compendium adds 177 books that were present in past Elder Scrolls games but not included in Skyrim."
http://www.nexusmods.com/skyrim/mods/22642/? - "This mod adds many buyable and findable books into Skyrim. They are lore friendly and fun. Some are imported and edited from previous TES games (Daggerfall, Morrowind, Oblivion), but most are not. Now you can read much more interesting journals, notes, stories, legends and even some lore friendly jokes!"
http://www.nexusmods.com/skyrim/mods/53340/? - "Little is known of the scholar Voryn Dreleth, yet his volumes survive still, scattered across the land. These rare volumes can be found in likely places, if you know where to look...."

http://www.nexusmods.com/skyrim/mods/12270/? - "This mod adds the book "ABCs for Barbarians" (previously found in Morrowind) to Skyrim, with updated scribbles to reflect the new items of Skyrim."
http://www.nexusmods.com/skyrim/mods/34673/? - Adds the book "Goodnight Mundus" to the game, a "lullaby released by Zenimax Online to promote "Elder Scrolls Online" - and this book adds the text from the lullaby to the game, to be found on sale randomly by booksellers."

BOOK SORTING

http://www.nexusmods.com/skyrim/mods/26913/? - "Organized Books is a lightweight mod I made when frustrated with the inconsistencies and improper sorting of the ingame books. ... For example, the book title The Wolf Queen, v4 has been changed to Wolf Queen, Vol. 4 of 8. Chapters of a book are also labeled as such; for example, A Dance in Fire, v1 is now Dance in Fire, Ch. 1 of 7."

http://www.nexusmods.com/skyrim/mods/32145/? - "Another Sorting Mod renames most items so they sort properly and thus provides a less tedious experience when managing your inventory. ... More consistent naming for Books, Notes, Letters, Orders, Recipes and such (some Notes are Letters now and vice versa)"

http://www.nexusmods.com/skyrim/mods/41587/? - "This mod renames and better sorts thousands of items in all categories (from books and soul gems to weapons and armors) in order to provide a cleaner, more organized, intuitive and useful listing of items in the game UI."

BOOKSTORES AND LIBRARIES
http://www.nexusmods.com/skyrim/mods/52248/? - A huge mod that among other features adds a "Bookset display case which sorts set books by their set, includes amazing retexture from 'Book covers Skyrim'".
http://www.nexusmods.com/skyrim/mods/33720/? - Adds a library that you can buy for the player home in Markarth. Has 32 bookshelves with auto-sorting for all vanilla books. Doesn't seem to work with Book Covers, though, as all books ended up in a mess when I tried it out.
http://www.nexusmods.com/skyrim/mods/55607/? - Adds a bookstore to Solitude.
http://www.nexusmods.com/skyrim/mods/32558/? - Adds a library northeast of Solitude with vanilla books as well as books from Daggerfall, Morrowind and Oblivion.

PLAYER HOMES WITH GOOD LIBRARIES
http://www.nexusmods.com/skyrim/mods/11158/? - "This is a mod that allows you to take Breezehome from it's humble beginnings as a little crap shack to a home worthy of a dragonslayer." One of the upgrades adds several bookshelves.
http://www.nexusmods.com/skyrim/mods/31375/? - Adds a "library which can be accessed if you have built the little "house/entrance" of your hearthfire home so that you can have your armory (or kitchen) in the east wing without sacrificing all those pretty bookshelves for your wonderful book collection". Obviously requires Hearthfire.
http://www.nexusmods.com/skyrim/mods/36191/? - Converts the "main upstairs bedroom (rendered useless by the bedroom wing) into a library. Besides the bookcases, the two wall shelves and the sideboard can also except books." Requires Hearthfire.
http://www.nexusmods.com/skyrim/mods/31455/? - Adds a library with "25 bookcases allowing 1350 books to be stored" to the player home in Solstheim. Requires Dragonborn.
http://www.nexusmods.com/skyrim/mods/31807/? - Adds displays for the Black Books to the player home in Solstheim. Requires Dragonborn.
http://www.nexusmods.com/skyrim/mods/53975/? - Adds a "small mage themed house above Winterhold, an Apothecary next to the Jarl's house, and smithing facilities next to the Frozen Hearth Inn." Has lots of bookshelves.
http://www.nexusmods.com/skyrim/mods/33939/? - Adds a player home close to Whiterun, with a "study/library with desk and 12 bookshelves".
http://www.nexusmods.com/skyrim/mods/50480/? - A "medium sized, adoption friendly player home located just outside Whiterun across from Honningbrew Meadery". Has lots of displays and "12.5 Bookshelves (550 Books)".
http://www.nexusmods.com/skyrim/mods/33661/? - Adds a "customizable & interactive home designed in one cell" in Riverwood with a small but cozy library.
http://www.nexusmods.com/skyrim/mods/51018/? - Adds a library to the to the Dragonsmount Player home mod. Requires http://www.nexusmods.com/skyrim/mods/49074/?.

MISCELLANEOUS

http://www.nexusmods.com/skyrim/mods/16463/? - "Changes the way skill books work, so that you may choose if and when you gain a skill from it. Upon activating a book, a menu will appear allowing you to select if you read the book and gain its knowledge at that time, or simply take it without reading."

http://www.nexusmods.com/skyrim/mods/9960/? - "This mod appends the skill improved to the name of every skill book, and loots skill books to your inventory without reading so you can save them for later."

http://www.nexusmods.com/skyrim/mods/21765/? - "Reading books now increase speech skill. Almost every book in the game is able to increase Speech skill progression towards next level twice."

http://www.nexusmods.com/skyrim/mods/57330/? - "This mod is a spiritual successor to dryblood’s mod; Read and Learn Speechcraft Speech. ... For every new book you read you will gain a flat amount of experience towards levelling up your Speech skill." Patches for Book Covers and Book Covers - Lost Library are in the works.

http://www.nexusmods.com/skyrim/mods/46088/? - "This mod makes it possible for everyone to add Audio Books (.xwm audio files) that will play when a player opens or activates a supported book, letter, scroll or note containing readable text."
http://www.nexusmods.com/skyrim/mods/10012/? - "Books which you have not read yet will have a visible glow. When you read a book for the first time, its glow will disappear."

http://www.nexusmods.com/skyrim/mods/51956/? - Adds HUD information, including an icon for books already read. Requires SkyUI and SKSE.
http://www.nexusmods.com/skyrim/mods/51496/? - "This is a mod which will give you access to a new MCM. Choose a category and see a list of all the known books in that category, as well as whether or not you've read it." Requires SkyUI and SKSE.
http://www.nexusmods.com/skyrim/mods/24879/? - Adds a "book called "Inventory at [Location]" and when you open it (then close it) it will display the contents of a container filled with (copies of) the items you placed around the house."
http://www.nexusmods.com/skyrim/mods/38000/? - "Following the writings of a lonely scholar the dragonborn must dive into a library of books and pits his/her wits against forgotten legends and lore to assemble the clues that will lead to great wealth, relics, and knowledge."
http://www.nexusmods.com/skyrim/mods/18258/? - Adds professions, including Scholar, which gives bonuses for reading books, learning spells and learning from a trainer.
http://www.nexusmods.com/skyrim/mods/57330/? - Adds a small portion of Speech experience every time the player character reads a new book. Has patches for Book Covers Skyrim and Book Covers Skyrim - Lost Library.
http://www.nexusmods.com/skyrim/mods/49457/? - Adds a quest and a house that can be used as a library.

NOTES: The list was inspired by http://www.gamesas.com/topic/1503208-mods-for-a-librarianbook-collector-playthrough/. I'll gladly add more mods to the list if you have any tips. The Misc. section is a bit messy so I might split it up with more categories. Thanks to Brakes Brakes Brakes, Moist_Typewriter, jackstarr, Siham, entreri3478, Glanzer, LukeH, myrrdin35, Dowaco and Merey for tips in the original thread.

A very useful list - thanks for compiling!