The original language specification, written by the authors, is now free online. Chapter 2 covers the whole language in a little over 40 pages.

https://archive.org/download/pdfy-MgN0H1joIoDVoIC7/The_AWK_P...

have a copy on my bookshelf! Didn't have a pdf though nice.

The gawk one is useful if you're into some of the gnuism specifics

Severely underrated comment.

Having relied heavily on the (unofficial, non-GNU) gawk manpage extensively (it's quite good), I instantly started learning very useful features reading the GNU docs. (I still need to fully internalise those). Yes, the full manual is very much better than the manpage.

(Also recommend The AWK Programming Language mentioned here, though I'd suggest the GNU manual adds to that as well.)