Hi cp,
here are my recommendations:
http://product.half.ebay.com/Amiga-Machine-Language_W0QQtgZinfoQQprZ1183610This one is a very short book, but it'll get your hands dirty with assembly and the Amiga hardware very quickly! You'll be playing sounds and doing graphics stuff by working directly with the hardware or AmigaOS libraries. It was a fun read, although it has a few typographical errors (I think because the author is German and the translator didn't do such a good job).
http://product.half.ebay.com/Amiga-Assembly-Language-Programming_W0QQprZ814212QQtgZinfoThis one you have to read, not only because the author's name is Commander
hahahaha, love it, but because it also teaches some proper computer science methodologies (like the software lifecycle, etc). It was a fun read as well and not too big a book either. Not so good as far as hands-on stuff, in other words the programs it gives you aren't anywhere close to as fun as the other book, but it will teach you the basics of 68k assembly pretty well.
Cheers
