The Witcher library guide

Books which must be purchased

 * The Book of Animals or Physiologus
 * Feainnewedd
 * Subterranean Plants
 * The Tome of Fear and Loathing, volume II
 * The Wonderful World of Insectoids

Books can be obtained for free

 * Ain Soph Aur (necessary for quest: A Mysterious Tower)
 * The Book of the Kiss
 * The Secret Gates (necessary for quest: A Mysterious Tower)
 * Specters, Wraiths, and the Damned
 * Swamp Monsters
 * The Tome of Fear and Loathing, volume I

Books obtained as rewards

 * Hymns of Madness and Despair (Quest: Diplomacy and Hunting)

Free books
For a complete list of all books which can be found, sorted by Act, see the Cheapskate's guide.