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, sizes, and overall variants.

Fallout 3
Pre-War books in the Capital Wasteland come in two different versions, green and rare blue. Aside from use as Rock-It Launcher ammunition, the green books are desired by the Brotherhood of Steel. Specifically, the Brotherhood Scribe Yearling, located within the Arlington Library, who will reward you with 10 XP and 100 bottle caps per book for contributing these to their archives.

Fallout: New Vegas
Pre-War books are a common item within the Mojave Wasteland, and can be found inside most occupied dwellings. 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). There are 9 varieties of pre-War books, with 4 different color patterns and 2 different sizes. Although most varieties of pre-War books are worth one bottle cap, a "relatively" rare variety of blue pre-War book is worth five.

In the add-on Old World Blues, you can take them to the Book Chute and have them turned to blank books used for making skill books.

Fallout 3
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:

Fallout: New Vegas
The rare blue variety is only known to reliably appear in these locations:
 * The Mining Office in Sloan. (1 book)
 * The Entrance Hall to Vault 22. (1 book)
 * An unoccupied Motel Room in Novac. (3 books)
 * Given to you by Ulysses if you didn't kill him during quest The End and ask him if he visited Hopeville after The Launch. (1 book)

Fallout 3

 * Yearning for Learning - green variant.

Fallout 3

 * Senior Scribe Yearling may sometimes not accept books from a location other than Arlington Library.
 * Crazy Wolfgang and a few other vendors may sell "false" pre-War books. Once you fully invest in his inventory, Wolfgang will begin to carry them. Books that can be turned in have a value of 1, a green cover, and show a special "Book" icon, while Wolfgang's books have a value of 5, a blue cover, and appear with the "Junk" icon in your inventory (not stacking with the regular ones). These "false" pre-War books can be found throughout the Wasteland, Point Lookout, and Mothership Zeta as well.
 * A blue and a green pre-War book can be found in the Operation: Anchorage add-on. Neither can be taken into your inventory.

Fallout: New Vegas

 * Pre-War books are much more common in the Mojave Wasteland, but cannot be exchanged for caps other than selling them to merchants.
 * Many huge versions of different books can be found inside the Ultra-Luxe, on shelves found in the hotel rooms and the penthouse suite.
 * Massive numbers of books can be found in the Lucky 38 Penthouse (around 200, and only a fourth of them can actually be picked up), the NCR Embassy (inside Dennis Crocker's room, his office and a small storage room), Vault 3, and Vault 21.
 * An entire bookshelf of pre-War books can be found in the barracks of the Mojave Outpost.
 * A full two shelves can be found in the Big Horn Saloon.

Bugs
Carrying more than 99 pre-War books and telling Scribe Yearling "just take all the books I have" will cause her to take only 99 books, yet give you caps and XP for all the books you carry. For example, carrying 110 pre-War books and giving them all to the scribe leaves you with 1100 XP, 11000 caps and 11 books left.