I recently published my ebook on GNU awk one-liners . It starts from the basics of awk syntax and then discusses one-liner examples. There's a chapter on regular expressions as well. The github repo has the details on how to get the PDF version, all the example files and code snippets used in the book, sample chapters as well the markdown source used to generate the PDF.
I'd appreciate your feedback and hope the books are useful. Happy learning :)