I am seeing this a lot recently. Just now it was trying to open the book BYOHHouseGuide, and all I see is "LOOKUP FAILED!" in the Book Text window of the book window pop-up. The pop-up shows other info for the book, like name, weight, and value. So, it would seem that the CK found the item correctly in the Hearthfire.bsa (which is included in the SkyrimEditor.ini area for BSAs to load). Wouldn't all of that be in the ESM/P file (not BSA)?

It works with books with prefix 00 (Skyrim.esp) but fails with 01 (like DLC1_WESC05Note in Dawnguard,esp).

What a pain! Any ideas?

EDIT: same thing happens opening perks added by mods other than Skyrim.esm, like DLC2CriticalHit. The Perk Description window has the error message.

Yes that data is located in the plugin file, not as a loose file. The only thing I could think of is that the issue may be related to strings/translation. Don't you have to do something to get DLC strings loading properly? Add an INI line, or extract them somehere or something? I forget, but it may be related.