Are you looking for theory regarding designing effects, performance or the psychology of magic? Also, is your focus close-up, parlor or stage? When you say illusionist, I think you are talking about the design of large stage illusions. Is that what you are looking for or is it something else?
Also, how long have you been doing magic and what sources (books, DVDs, etc.) have you been learning from? Your level of experience in learning and performing magic will have an effect on which book would be best for you.