The Super Affiliate Handbook by Rosalind Gardner is a comprehensive guide to making money with affiliate programs, and offers a lot of solid information. Having run several successful affiliate sites myself, I can confirm that the information in this book is accurate and extremely useful.
If you are already running your own profitable affiliate site, you may already know or at least be familiar with many of the concepts that Rosalind Gardner covers in her book. But you will still find the book worthwhile, since it has 280 pages worth of information and you are bound to find a few new ideas, regardless of your level of expertise. For example, I got a lot out of the chapter on gauging demand the analyzing the competition. Overall, this is an excellent guide for anyone interested in running affiliate marketing sites.