GNU YACC (Bison) comes with a very nice info-file (.info) which documents the YACC syntax very well. It mentions Lex only once, but otherwise it's very good. You can read .info files with Emacs or with the very nice tool 'pinfo'. It is also available on the GNU site: BISON Manual.
Flex comes with a good manpage which is very useful if you already have a rough understanding of what Flex does. The Flex Manual is also available online.
After this introduction to Lex and YACC, you may find that you need more information. I haven't read any of these books yet, but they sound good:
By Charles Donnelly and Richard Stallman. An Amazon user found it useful.
By John R. Levine, Tony Mason and Doug Brown. Considered to be the standard work on this subject, although a bit dated. Reviews over at Amazon.
By Alfred V. Aho, Ravi Sethi, Jeffrey D. Ullman. The 'Dragon Book'. From 1985 and they just keep printing it. Considered the standard work on constructing compilers. Amazon
Thomas Niemann wrote a document discussing how to write compilers and calculators with Lex & YACC. You can find it here.
The moderated usenet newsgroup comp.compilers can also be very useful but please keep in mind that the people there are not a dedicated parser helpdesk! Before posting, read their interesting page and especially the FAQ.
Lex - A Lexical Analyzer Generator by M. E. Lesk and E. Schmidt is one of the original reference papers. It can be found here.
Yacc: Yet Another Compiler-Compiler by Stephen C. Johnson is one of the original reference papers for YACC. It can be found here. It contains useful hints on style.