For awk, start with the book by Kernighan/Aho/Weinberger. The PDF of the first edition is freely available on archive.org, and a second edition has been announced. Chapter 2 of the first edition is only 40 pages, and describes all language features.
For sed, the best resource is likely the free "UNIX Bookshelf" from O'Reilly that can be found online; this contains a book specifically focusing on Sed and Awk.
https://www.awk.dev/