The major textbook is Single Variable Calculus: Concepts and Contexts, 3rd edition, by James Stewart. Brooks/Cole 2005, ISBN 0-534-41022-7.
The full version of the Stewart's Calculus includes the Math 21a material and is a good buy if you're thinking about taking. It has ISBN 0-534-40986-5.
(The links are to the Wikipedia Booksources pages, which give not only information about the book in various catalogs but links to places to buy the book.)
I definitely understand students' interest in saving money. Unfortunately, I cannot recommend using the 2nd edition of the book for the course. The examples and explanations have been changed--we hope for the better--and since there are required reading assignments we would like to make sure you're reading the same book we are. Also, 25% of the problems in the 3rd edition are not in the second edition, and worse there are probably reorderings of problems. Anyone using the 2nd edition would be responsible for synchronizing the editions for every homework assignment. That might prove to be a huge headache.
Read Sections 5.1 and 5.2 of the book and answer the pre-class reading question. This can be found on the course web site, under "Q&A 2.0"
Students in TTH sections will also need to read Section 5.6 and answer that survey question.