Of all the filters manufacturer, B+W are the only one to have put online a coherent guidebook on their huge filter inventory. I've got some printed Kodak guides, but they do not get as exhaustive as this. Whereas Kodak tends to give general characteristics, the B+W guide characterizes precisely every single filter, so you finally get to know the difference in purpose between a KR1.5 and a KR5 filter (and no it's not a full conversion filter for tungsten film in daylight!).

Their classification is logical, and many important distinctions are drawn. Unlike the rather obscure Kodak/Wratten numbering schemes, B+W's is extremely systematic and regular. Even if you use other brands, this guide book will help you map clearly the purpose of the various light balancing, contrast, colour correction, and other filters. Finally, precise transmission curves for ALL filters are drawn, and a very tight product table helps figuring out availability (it's surprising how many filters are still made in Bay I for instance).