Post-AmigaOS3 documentation, let alone a formal publication, is nearly nonexistent. The 3.0 or 3.1 manuals (Workbench, AmigaDOS, ARexx) are probably your best source for detailed user-level information, with the caveat that anything introduced in 4.x won't be covered.
Also consider "AmigaDOS Reference Guide, Fourth Edition" by Sheldon Leemon and "Denny Atkin's Best Amiga Tips and Secrets" which were both published by Compute Books.
A good amount of stuff was published in the AmigaOS2.x era, and most of it will still be valid.
4.x does have the Documentation drawer on the SYS: partition, but the quality, style, and completeness of the info in there varies greatly (and some of it hasn't been updated since the 4.0 pre-release!).
