ToastyTech has images, descriptions and miscallenous trivia for most popular microcomputer GUIs. Seems to lack a few (NeXT etc.) but is still a really terrific read. Also, you can now read scans of the 1971 and 1979 versions of "'How It Works': The Computer" books, intended as an ease-into guide for the computer-illiterate.

As a history buff I've always found older computers fascinating, both the computers themselves and the social phenomena they represent. If you're also interested in such things, make sure to read Fire in the Valley: the making of the personal computer, which is an astounding read which delves (in interviews, pictures and background stories) into the various aspects of the early PC era. Another interesting read is Steven Levy's Hackers: Heroes of the Computer Revolution (which my brother Mickey bought me as a birthday present couple of years back. Thanks, man!)