In-depth technical information

• Atari STE FAQ compiled by The Paranoid / Paradox
• Blitter manual
• Keyboard Protocol (english version)
• Line A documentation
• Memory Map for Atari ST,STE,TT and Falcon
• MFP MK68901
• The Atari Enhanced Joystick Ports FAQ
• The authoritative guide to the Falcon video hardware
• YM2149 Programmable Sound Generator

There are a lot of Atari hardware documentation at the Atari Document Archive