Asimov repository is the place to go on internet to find apple stuff. There are disk images, original documentation, ROM images, emulators and so on.
Asimov repository is here : ftp://ftp.apple.asimov.net/pub/apple_II
Jon Relay's Apple II Info Archives contains useful stuff that can also be found in reference manuals, except for the fabulous I/O Memory page that summarizes input/output of all Apple II models in one big table :