I'd be a great idea to write a modern Atari assembly language book. It would, of course, cover the basics of assembly language. From there it can begin to cover the hardware and how to exploit it with commented examples. Player/missile graphics, software sprites, multiplexing players, sound, DLIs, VBIs, double buffering, what you can do when you move the OS to RAM, etc.
Sort of a modern De Re Atari...