Scott Jenning does a great job of covering the basics of MMOGs, introducing the reader to a wide array of topics: Class role, group teamwork, endgame play styles, professions, and etiquette.
As an intermediate player (~60 days played in WoW, a level 60 and many level 40 alts), I still found lots of new ideas, as well as lots of confirmation about things I had suspected, but was unsure of.
If you want to find help getting into your first MMOG, this is an amazing book. If you are an experienced player, this is still worth the read for the entertaining anecdotes and the few tidbits that you might not yet know.