About the book (from Wikipedia):
The Jātaka tales (Sanskrit: जातक, birth history’) are a voluminous body of literature native to India concerning the previous births of Gautama Buddha in both human and animal form. The future Buddha may appear as a king, an outcast, a god, an elephant—but, in whatever form, he exhibits some virtue that the tale thereby inculcates. Often, Jātaka tales include an extensive cast of characters who interact and get into various kinds of trouble – whereupon the Buddha character intervenes to resolve all the problems and bring about a happy ending.
In Theravada Buddhism, the Jātakas are a textual division of the Pāli Canon, included in the Khuddaka Nikaya of the Sutta Pitaka. The term Jātaka may also refer to a traditional commentary on this book.