> The problem with all the Atari's in my mind was that they were not dev-friendly machines.

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.