Here's some wonderful new material provided by our friend Harry Stewart, and scanned by me.
Undated, circa 1978? The author is not noted on the document. This is certainly the earliest technical description of the Atari 8-bit computers that I have seen. Version A is charmingly handwritten. A lot of this material will be expanded to become the Atari 400/800 Hardware Manual.
Second draft of the manual for the "Atari A1200" computer, which would be renamed Atari 1200XL prior to release. Manual dated November 30 1982, by Robert A. Peck. It primarily shows the differences between the 1200 and its predecessors, the Atari 400 and Atari 800. Near end end of the document, you'll see a memo by Harry Stewart (dated January 10, 1983) with suggested edits to the document.
Manual for LNBUG version 5, the 6500 Development System by Larry Nicholson of Cyan Engineering. The manuals for v2 and v6 were previously made available, though I don't think the software has come to light.
This is the earliest version of the Atari 400/800 Operating System Users Manual that I am aware of, dated August 15 1980 (there was already a November 1980 version online. The official public version wasn't released until 1982.) Lots of hand-written notes. Some of the document is ... rough.
Share the interesting things you find in these documents! /|\