a website that had John Chowning's out-of-print "FM Synthesis: Theory and Applications" text scanned as a PDF. As the site looked like it hadn't been updated in 5 years, I uploaded the file to my own hosting in case something happened to the original. No idea who did the original scan, but many thanks whoever you are.
If you don't know who John Chowning is, he is none other than the inventor of FM synthesis which went on to become the sound of the 80's and sell Yamaha a whole lot of DX-7's. Unfortunately, FM Synthesis isn't quite as intuitive as good old subtractive synthesis, and that's where this text comes in. More or less considered to be the bible of FM synthesis, the text may help to clarify things a bit for you if you're struggling.
GO GET IT!