I would have to say that the best technical documentation I've ever come across has to be the Commodore User Manuals for VIC-20 and Commodore 64. Coupled with Compute!'s Gazette I was able to launch a career in BASIC programming. The user manuals completely documented the entire architecture of the KERNAL and BASIC system, as much as any programmer could need to do all sorts of things. Kudos to Commodore on documentation, definitely a lost art, and never found in Windows-based apps again.
Similarly, the manual that came with the original ZX Spectrum rubber key version was awesome, with a complete description of Sinclair BASIC and all the Z80 assembler op codes for good luck. The 48k+ manual that followed was nowhere near as useful.
