Emil Artin's little book on the gamma function, currently out of print. The most special of the special functions, the gamma function's interesting properties and relationship to the number-theoretically critical zeta function make it an intriguing topic for a mathematics student to tackle. Artin's book is delightfully written, and should be accessible to an advanced undergraduate student or beginning graduate student. Highly recommended to the mathematically inclined (if you can find a copy).