Hi - if you're referring to the career specific books, there are two ways your sim can get them.
Opportunities - if you don't have a career-related opportunity already, you might get the option to read a book to improve your job performance. If you accept the book is dropped into your inventory. The Sims Wiki has information about opportunities
http://sims.wikia.com/wiki/Opportunities, although the lists don't look particularly complete to me. The Prima Guide has more information.
Some careers also have journals (I say some, but I can only think of the medical career). Reading journals is one of the metrics to advance. A new book is dropped into your inventory after each shift.
Does that help? If you have more specific questions come back to us.
Nala.
