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:
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.