by SaulGellerman (1992) ISBN 0452270235
This is the best book on motivation I've ever read. Gellerman is a psychologist, and he goes into great detail about why some people are motivated differently than others. (This may be enlightening to those who are used to dealing with other technical or professional people, who often have similar motivations).
Some parts of the book are advice for people who want to become top executives, which may not seem relevant to people who are more interested in technical careers. But the basic psychology is much more thorough and rigorous than what you'll find in a lot of business books.
For more on the history of motivational research, see his earlier book MotivationAndProductivity.