Ian,
There are a ton of books out there on CBT, it's theory and it's practice. To get a basic idea of what it's about, I would recommend doing a google search and see what comes up.
Here's what I found:
http://www.nacbt.org/whatiscbt.htm
Another option would be to check out a book on therapy techniques from the library, most of which would include a section giving a good introduction to the theory and technique.
Aaron Beck's "Cognitive Theory of Depression" is the seminal work on the subject but not the most current.
If you're looking for a book to get you started in being able to apply the techiques in your work as a peer counselor, one place to start would be a book called "Taking Control of Your Moods and Your Life: a thoughts and feelings workbook" by McKay, Davis, & Fanning. It is essentially a treatment/self-help manual.
As much as some of my heroes, such as Irving Yalom, would caution against "manualized" therapy, I have found that using a workbook as a guide makes for a very helpful adjunct to a more client-directed approach. For a therapist who is new to the field, using a workbook with a client is a great way to get familiar with the concepts as you introduce them to the client. Eventually, you probably won't need to refer as much or at all, but it's a very helpful tool for getting started.
Hope this helps and best wishes for your studies!
-FT