XView and OpenLook are included in some form on probably all Linux distributions these days, but sources are available at export.lcs.mit.edu and many other sites; archie will help you locate a site nearby. Binaries for Linux are available at Sunsite and its mirrors.
The manual page for XView covers much of the basic information, but not the details for each component. A good reference is necessary to get the details for all the component attributes.
As for printed documentation, two volumes used to be available from O'Reilly, The XView Programming Manual Volume 7, and its companion, The XView Reference Manual. In mid-1996, O'Reilly declared these two volumes out-of-print. If you're lucky, you might be able to find a bookstore with a copy, or perhaps someone at a local computer-fest may have a copy stored away.
On the brighter side, Darwin Open Systems sells a CD-ROM that includes the complete text of both O'Reilly volumes (in PostScript and PDF formats), other OpenLook materials and sample programs.
One tutorial book that might still be in the publisher's warehouse is Practical XView Programming, by Kenneth Bibb and Larry Wake, John Wiley and Sons.
More information about the OpenLook and XView CD-ROM is available at http://www/uunet.ca/darwinsys/ or by e-mail to firstname.lastname@example.org, or see the review of the CD in Linux Journal, December 1996, Issue 32.
Finally, there are two newsgroups that cover XView: comp.windows.openlook for OpenLook related questions and alt.toolkits.xview for XView programming questions. Traffic is usually light on both, so it's easy to scan through the few articles that show up.
Source code and compiled versions of the sample program and a “Notice” pop-up program are available at http://www.balr.com/.