The user documentation for the different versions can be found here:
A brief getting started guide and tutorial for end-users and non-programmers is 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:
X Series Volume 3: X Window System User's Guide, 3rd Edition (1990, covers X11R3 & R4)
X Series Volume 3: X Window System User's Guide, OSF/Motif Edition (1990, covers X11R4 & Motif 1.1)
X Series Volume 3: X Window System User's Guide, OpenLook Edition (never published)
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 X Power Tools, but only a small preview of it is available for free online at this point.