The user documentation for the different versions can be found here:

 * [[http://ftp.x.org/pub/X11R6.9.0/doc/html|6.9.0]]
 * [[http://ftp.x.org/pub/X11R7.0/doc/html|7.0]]
 * [[http://www.x.org/releases/X11R7.5/doc/|7.5]]
 * [[http://www.x.org/releases/X11R7.6/doc/|7.6]]
 * [[http://www.x.org/releases/X11R7.7/doc/|7.7]]
A brief getting started guide and tutorial for end-users and non-programmers is [[UserDocumentation/GettingStarted|here]].

O'Reilly & Associates have also made freely available online some of their classic X Window System manuals. These are a bit older, dating from the X11R3, R4, or R5 eras, and applying to the commercial Unix versions of the time, but may still have some useful
information in:

 * [[http://www.archive.org/details/xwindowsystem03quermiss|X Series Volume 3: X Window System User's Guide, 3rd Edition (1990, covers X11R3 & R4)]]
 * [[http://www.archive.org/details/xwindowsytemosf03querarch|X Series Volume 3: X Window System User's Guide, OSF/Motif Edition (1990, covers X11R4 & Motif 1.1)]]
 * [[http://www.oreilly.com/openbook/openlook/|X Series Volume 3: X Window System User's Guide, OpenLook Edition (never published)]]
 * [[http://www.archive.org/details/xwindowsystemadm08muimiss|X Series Volume 8: X Window System Administrator's Guide, 1st edition (1993, covers X11R5)]]

O'Reilly has published a much more recent and up-to-date user guide [[http://books.google.com/books?id=V1ZBeNJIx7UC|X Power Tools]], but only a small preview of it is available for free online at this point.