Thanks to both of the responders. The book by Caleb Finch is a great reference book and I have put it on reserve in the library. It is bit old (1994) and too encylopedic to be used for an undergraduate textbook. I have used the book by Leonard Hayflick, and I really liked it even though it was written more for a lay audience and not senior biology students. Regrettably, it is no longer in print and available in the numbers I need for a class of 30 students. Perhaps I'll just have to take the time to write the book that I want!