The primary documentation is man pages available after installation.
Verilog-Perl manpage - Combined HTML of manpages for searching or printing.
Vppreproc and the preprocessor were the topics in a paper by Wilson Snyder <email@example.com> for the 2010 Boston Synopsys User's Group.
VPAssert was presented by Duane Galbi at the 2000 Boston Synopsys User's Group meeting. The paper, titled Synthesizable Watchdog Logic: A Key Coding Strategy for Managing Complex Designs won the Best Technical Paper award. (Note VPAssert used to be named VPM, which is how the paper refers to it.)