At the risk of sounding like a corporate shill, I'll first suggest going to Bedford/St. Martin's Teaching Central Professional Resources site. They will send you free sourcebooks. Here's a list of some of the ones I like:
- Assessing Writing edited by Bryan Huot and Peggy O'Neill
- Feminism and Composition edited by Gesa E. Kirsch, Faye Spencer Maor, Lance Massey, Lee Nickoson-Massey, Mary P. Sheridan-Rabideau.
- Teaching Developmental Writing edited by Susan Naomi Bernstein
- Teaching Composition edited by T. R. Johnson
- Assigning, Responding, Evaluating by Edward M. White
Other texts that might be of interest:
- The Subject is Writing edited by Wendy Bishop. This book focuses on classroom practices.
- Cross-Talk In Comp Theory: A Reader edited by Victor Villanueva. This is a nice overview of the field. Obviously, it doesn't cover everything, but it's a good starting point. Personally, I'd get it through the library and/or get the table of contents and find the original articles in your library databases. It's a nice resource to have on your shelf if you want to buy it, but there are more economical ways to get your hands on the contents.
- The Writing Teacher's Sourcebook edited by Edward J. Corbett. Same advice as for the previous book. This one does focus more on teaching.
- Engaging Ideas by John C. Bean. I tend to suggest this book for people outside of English studies, but it does have a lot of good practical advice in it.
Of course, I need to mention Rebeccah Moore Howard's online bibliographies. Her list for Writing Program Administration is here.
I know I've left things out and I'll probably remember the "perfect" text as soon as I post this, but this is a good start. Everyone should feel free to add more suggestions in the comments.