http://www.grymoire.com/Unix/Awk.html - "Awk: a Tutorial and Introduction", by Bruce Barnett. Reputed to be one of the more easily readable awk references.
http://www.opengroup.org/onlinepubs/9699919799/utilities/awk.html - "awk: pattern scanning and processing language - Commands & Utilities Reference", The Single UNIX Specification (includes a nifty section defining the precise grammar acceptable as awk source code)
http://www.gnu.org/software/gawk/manual/gawkinet/ - "Gawkinet: TCP/IP Internetworking with gawk", a relatively obscure but /very/ nifty document explaining how to make impressive use of gawk's networking facilities (though Plan 9 users are liable to sniff that their OS offers a proper /net entry in the filesystem, making such language-level extensions ugly and unnecessary)
The A-Z of Programming Languages: AWK - "Alfred V. Aho of AWK fame talks about the history and continuing popularity of his pattern matching language." (Naomi Hamilton, in Computerworld; 2008-05-27 09:43:32)
a trio of articles written in 2000-2001 by Daniel (not Arnold) Robbins (the first article was recently updated, which is probably why it appeared on somebody's feed radar). Parts 1, 2, 3.