The GNU Awk manual has several practical examples in it - as manuals should. (Looking at you, nearly all man pages in linux distros.)

The original 1988 book The AWK Programming Language by A, W and K is, in my opinion, one of the finest pieces of documentation ever written, on any subject. A joy to read and be instructed by.

I have occasionally dipped into this book for instruction and consider it a freely accessible reference:

Unix Text Processing, Dougherty and O'Reilly, 1987 https://www.oreilly.com/openbook/utp/

Cherry pick chapters like those on the Shell and AWK, and avoid those on troff, macros etc unless specifically interested.