But starting with De Re Atari by Chris Crawford in 1982, a lot of development material became available. Compute! had a great line of books, including Mapping the Atari.
It was shame it took so long for that material to appear because the Atari 8-bit have a rather elegant OS, especially compared to its contemporaries.
That was true in the early days of the 400/800, but by 1982 when the 1200XL was released (a few months ahead of the C64) they'd corrected themselves. The board schematics and assembly source for the ROM was a book you could buy at the dealer, and sources like De Re Atari and Compute Magazine had collated all the relevant details of the handful of ASICs such that people could start playing weird tricks.
It wasn't Woz's Red Book (neither was Commodores documentation), but it told essentially the whole story of the devices down to the MMIO level.
(I remember reading it end to end as a child laid on my parents' bed because the light was better in there than in my room, shortly followed by developing the programming addiction that has stuck with me the rest of my life ;)
It didn't cover arm2 assembly, but my parents bought me an extremely good book that did - and described the chip architecture itself in detail as well.
I only touched a Commodore at a friend's place to play games on it, but it sounds like they also understood hobbyists :D
Commodore machines came with a rather hefty serial bound book that introduced you to programming and gave you a memory map of the hardware, important PEEKs and POKEs.
Atari's came with trade secrets.