My cousin d!rt!e just sent me a link from amiga-news.de where they posted an announcement of a new version of the Extra-Strong Assembly 1.18.
I never heard of that one ... did U ?!?11Questionmark
Never heard of the ESA before. It's amazing that he (she?) is still updating this in 2008! I think I'll stick to ASM-One 1.48 myself, though the ESA does look useful.
Speaking of assemblers; anyone know of any cross assemblers for 680x0 and AmigaOS on Windows, or perhaps just some simple editor with related syntax highlighting? Have to say even a standard edit box in Windows is a lot more comfortable than the ASM-one editor.
Zero: you can check vasm from Frank Wille, it's near 100% compatible to PhxAss. The only thing it lacks is REPT/ENDR support.
Source code is available but you may need GCC and GNU make to compile it.
Assembler: my favourite: PhxAss (http://aminet.net/package/dev/asm/PhxAss); a lot of people likes Asm-Pro (http://aminet.net/package/dev/asm/AsmPro).
Documentation: if you are not new to assembly programming, all you need is the M68k Programmer's Reference Manual from Motorola (item code: M680000PM/AD) or something like that; if you are new to assembly programming, then you also need some guide that teaches you how to code (and think) in assembly - look around the web. Oh, well, I said "all", but indeed also the MC680x0 User Manuals are useful for advanced programming.
As regards the Amiga chipset and OS, you already have the RKMs, which is a good starting point. However, those don't cover the AGA chipset (for which home-made documentation is available on the Internet).