Astronomy Hyper-text book

An online textbook in HTML format, hosted by the University of Oregon Physics Department

Handbook of Space Astronomy and Astrophysics

By Martin Zombeck, Smithsonian Astrophysical Observatory; online in HTML with hyperlinks added, hosted by The NASA Astrophysics Data System

Mars

By Percival Lowell, 1895; online in illustrated HTML from Bibliomania

On the Heavens

By Aristotle, c. 350 B.C.; HTML format from The Internet Classics Archive Space Physics Textbook - Hosted by The Space Physics Group of Oulu, Finland