Reading Usable Help
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Gordon R. Meyer
TIP database of instructional theory
The Theory Into Practice Database is maintained by Dr. Greg Kearsley and provides descriptions, cogent examples, and references for over 50 theories related to learning and instruction. (Two in particular, Minimalism and GOMS have been keystones in Apple's documentation for several years now.)
It's interesting to review all of these theories, then pick up a manual or help system and see if you can guess what influenced its design and implementation. Of course, some style guides, templates and designs are now so well established, and modified by practical or accidental divergences, that the theoretical underpinnings are well hidden.