After finding and purchasing all the books, a cutscene involving bookshop owner Sofia Sartor and Ezio Auditore is unlocked at the Assassin hideout.
Maps
Maps revealing the locations of the treasure chests in Constantinople and Cappadocia can also be bought, as well as a map revealing the locations of Ishak Pasha's Memoir Pages.
Buying every book available from the Book Shops in Constantinople and Cappadocia, along with finding all the hidden books from Polo Memories, unlocks the "Sage" achievement/trophy.
The December 20 2011 patch introduced a bug that prevents the game from awarding Sage after collecting all the books. If a subsequent patch does not fix this bug, clear your cache before obtaining the last book.