- Recipe selection on APUG and digitaltruth for a brief overview of common formulas. Don't get carried away with these recipes, quite a few of them are outdated and won't work as advertised with new film stock.
- Mees, The Theory of the Photographic Process. This book is very old (think Forties) but very informative, and best of all, you can download it legally and for free.
- Haist, Modern Photographic Processing. Despite its much newer date of publication, it offers quite outdated info on many subjects. Kodak would have stomped heavily on Haist's toes if he would have published more recent discoveries it seems. Still, B&W processing hasn't made all that much progress in the last 50 years, so his book is (IMHO) a very valuable and accessible resource. Although its main focus is B&W processing, it delivers the best explanation of color processes I have seen so far.
- Anchell&Troop, The Film Developing Cookbook: a very short and concise resource, and the only book which looks at fairly modern recipes, including Crawley's formulas and necessary modifications to make a dev work well with T-Grain emulsions. Also the only book which describes Phenidone's properties compared to Metol.
- Anchell, The Darkroom Cookbook: a much larger volume than the Film Developing Cookbook, mostly a big recipe collection.
If you have a very tight budget, get at least the free book from Mees and Anchell&Troop's Film Developing Cookbook.