I'd like to add the Spell Tomes from Dawnguard and Dragonborn to the leveled lists used in the main game, so I don't have to start the DLC to get the spells. However, I don't even know how to mess with the leveled lists. Is there a tutorial somewhere that would allow me to add these Spell Tomes to the appropriate lists?

If you Load up the Dawnguard or Dragonborn .esm in the Creation Kit, then take a look at "LeveledItem" under the "Items" tab on the left, you will see all the leveled item lists used to generate dungeon loot. In particular, editing any of the "LItemSpellTomes..." will change the tomes that appear in places where tomes already appear in the gameworld. You should be able to right click on any of these LeveledItem lists and select "edit," then rightclick in the list to add "New" items to the leveled item list which will possibly be chosen whenever that leveled item list is accessed. You will then have to choose the tome you want in the dropdown box while your new line is highlighted. Most of the DLC spells start with "DLC..." (for instance, DLC1SpellTomeConjureBoneman). Save your changes to a new plugin and that should do the trick!

Although, if you want the spells to work without having the DLCs loaded, it will be a lot more complicated, as you will have to copy over all the magic effect files, possibly even the meshes and textures related to the spell if they are not native to Skyrim.esm--something I wouldn't be able to help you with.