Pre-War book (Fallout: New Vegas)

A Pre-War book is a book that has not been destroyed, burned, or otherwise rendered unreadable. It is a fairly rare item in Fallout 3, however in Fallout:New Vegas the books are extremely common by comparison and come in a variety of colors.

Fallout: New Vegas
Pre-War books are a common item within the Mojave Wasteland, and can be found inside most occupied dwelling. They are one of the first miscellaneous items the player is likely to encounter in bulk: 28 of them are located inside Doc Mitchell's House.

Compared to their use in Fallout 3, Pre-War books in Fallout: New Vegas serve no real purpose, other than decoration.

Fallout 3
Pre-War books are desired by the Capital Wasteland Brotherhood of Steel. Specifically, the Brotherhood Scribe Yearling, located within the Arlington Library, will reward you with 10 XP and 100 bottle caps per book for contributing these to their archives. See Yearning for Learning.

Locations
G.E.C.K. shows that there are 110 Pre-War Books ("BookGeneric01") in the game, more are added with add-ons. The links below often tell you where to find them: