This has lots of great amiga programming references and links....
Utilitybase.com (Opps, just noticed SamuraiCrow mentioned that too)
And I'm afraid I have to disagree with the Abacus Amiga C programming guide. (But I do like the Inuyasha avatar) I found Abacus incomplete and hard to follow. I would recommend finding a good book for beginners on C programming, then incorporate amiga specific code later as you get better with C. Other opinions will vary.
Plaz