The first three hypnosis books are the most important in this group.
The New Encyclopedia of Stage Hypnotism (1st book, below), is considered the bible of Hypnosis. It covers just about everything you need to know about establishing the state of hypnosis whether you are a clinical hypnotist or a stage/street hypnotist. It should be the foundation of every hypnotist's reference library.
Hypnotherapy (3rd book, below) is considered the definitive reference for clinical hypnosis via the Elman school of thought. It thoroughly details the Elman approach to using hypnocounseling and hypnotherapy for clients.
Sports Hypnosis is an excellent source of information for hypnotists who specialize in sports hypnosis. By reviewing various case studies, the author teaches several approaches to address issues commonly faced by athletes.