[JKCalhoun]

"After the 400 and 800 launched, power users awed by Star Raiders proved eager to flex the machine’s advanced capabilities. But Atari, following its closed model with the 2600, had never intended to spill the secrets of the HCS architecture outside of special agreements with contracted developers. Crawford recalls, “There were about half a dozen people I knew who’d been bugging me for that information, and I had told them, ‘No, I can’t tell you anything.'”

Ironic that it was the Atari that seemed like the "closed system" then. I had a 400 with the horrible membrane keyboard (hey, it was cheap) but there was no documentation on how to program it outside of BASIC.

[awful]

"Mapping the Atari"; Ian Chadwick, 1983. A must have, so very sought after.

[rbanffy]

Since I was an Apple II person, I had What's Where in the Apple II.

Which is, surprisingly, available: https://www.lulu.com/en/gb/shop/phil-daley-and-william-f-lue...

[gramie]

For Commodore, we had the Commodore Inner Space Anthology, put out by the wonderful people who published The Transactor magazine (http://csbruce.com/cbm/transactor/).

[protomyth]

That book was amazing. The Atari Assembler cartridge and magazines completed the picture of what you could do with the Atari 8-bit. I know there was another book I loved, but I cannot remember its name. I have them in a storage garage along with my 400 and 130XE.

I had friends who had the Action language cartridge but I never got to play with that one. Apparently there was information there about the machines internals.

[buescher]

Before that, De Re Atari.