If you can find a copy, I recommend a book called Complete Amiga C. I can't remember the specifics of exactly what it covers but I remember it included general C topics (language construct, syntax, stdlib, etc) as well as topics specific to Amiga coding.
Quick Google search for details on the book has just thrown-up this thread btw:
http://www.amiga.org/forums/showthread.php?t=25144if you're lucky "Spudley" might still have his copy
Also, Aminet has the famous Amiga C Manual:
http://aminet.net/dev/c/ACM.lha . This is mainly an Amiga specific programming guide, so if you haven't learn C yet then it's probably not a good place to start. Also, it is based on v1.3 Amiga libs, so no 3+ specific info. But once you've grasped the general Amiga system library/API concepts you can simply read the latest Autodocs from the AmigaSDK for newer details.
hope it helps
