Designing & Writing Online Documentation: Helpfiles to Hypertext, by William K Horton. John Wiley & Sons 1990, ISBN 0-471-50772-5.
Any book which constantly quotes people like Alan Kay and Ted Nelson (the original Hypertext Prophet, whose Project Xanadu has been two decades of trying to make WorldWideWeb happen) can't be all bad!
I like this book: It has lots of good advice, quotes lots of useful numbers with research references to back them up, and is obviously written by someone who knows and cares about the field. The book is more general than the title suggests: It contains useful advice about designing just about everything visible to the user, on everything from glass TTYs to bitmapped multi-window graphics displays.
This book won't tell you anything about the nuts and bolts of coding in any particular user interface toolkit, however: You'll want another reference for that.
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