So I've beaten the game and got most of the side quests done and now I'm just challenging myself. Does anyone know of any rare books I can find around Skyrim?

the biography of Barenziah series is good.

I personally liked these:

Mixed Unit Tactics
Three Thieves
Cabin In The Woods
The Argonian Account

My favorite is A Marksmanship Lesson. Simply brilliant.

The ′A dance in Fire′ series, and ′The Real Barenziah′ series and of course ′The Wolf Queen′ series.