As Bikther said, it's good for some things, not so good for others.
If you're an experienced MMORPG player, looking for a quick reference to figure out where to find gathering resources, zone maps, etc, it's not bad.
It's got a nice listing of the equipment in the game, broken down by slot (head, back, gloves, etc), material (cloth, leather, chain, plate), and quality (epic, superior, good, standard, inferior).
Each zone has a map, a listing of the major NPCs (if applicable), areas within each zone, resources, a listing of the quests in or involving the zone, etc.
For a total and complete newbie to the MMORPG world, it is, imo, absolutely fantastic. It has a
great listing of common terms (afk, brb, lfg, etc), it lists all the in-game emotes, gets into setting up macros, discusses grouping, guilding, faction, has a good explaination of the different types of quests, each class gets its own section, with the class skills/spells/abilities, as well as the talents. I know it's fantastic for the compete and utter newbie, because I got my parents (both in their mid 50's) hooked on the game, and they absolutely love it. It has made their introduction to the MMORPG world so much easier than it otherwise might have been. And helped my sanity somewhat too.
They aren't having to ask me every little thing, they look a lot of it up in the book first, then ask me for clarification afterwards, if there's stuff they are still unsure of. Although, they wish the print was a little larger, but that's an understandable complaint of someone that's a bit older, or those that have vision problems. The majority of the MMORPG world won't have a problem with it.
The class skill listing is ... eh so-so. For the stuff it has right, it's great, because it's got them all laid out nicely with their descriptions and such, so you can compare and figure out how you want to spend your points a little easier than trying to figure it out in-game on the three different tabs. Unfortunately, as Bikther said, some of the info is incorrect, because it was based on information from Beta.
If you are willing to accept it as what it is, a fairly decent quick reference manual for the experienced player, a nice basic starting guide for the newbie, then it's good. If you are expecting it to be a massive spoiler, telling you precisely what's where and how to do things to get to the end game, etc, then you won't like it.