Back in the days (1991 or so...) I started with Asm-One which came with a small binder including some basic tutorials. Not that that old version of Asm-One would be any good today (checking out the more current and free versions would be a good idea), but such a manual could help you with 1st steps.
Once you passed that you will need a book on the 68000 and the RKMs (any Dev-CD should do here if you can't source the printed version). On top of that you should try to source some of the more scene-oriented mags from the 86-94-era as these often had demo-tutorials.
All that would cover OCS, information on AGA might be bit harder to source.
