Of course a book isn't the only way to learn regex. What really makes you learn is practice and a good tutorial, apart from frequent use of "info sed" for help on sed regexes, and "man 7 regex" for info on POSIX regexes.
BTW, in the info page for sed I find a page linked: http://sed.sourceforge.net/grabbag
And google for "sed one liners", it's a very well known doc with the more common sed tasks. There's also the annotated version: "sed one liners explained".