I know that the original request was for online reading about awk, but I can't help adding my recommendation for the offline dead tree version of "The awk Programming Language" by aw&k. I resisted buying it for years, on the grounds of its extraordinary price on Amazon, succumbed eventually, and have never regretted it. The book is a little miracle of clarity in a confused world.
And if you don't want to buy that, consider "The Unix Programming Environment" by Kernighan & Pike - chapter 4 covers awk, and the rest of the book covers all of the bits of Unix you needed to know until the lunatics took over the asylum.
