Mysteries of the Moonsea is WoTC's newest addition to the Forgotten Realms Campaign Setting. It is mostly for DMs, and to use it, DMs will need the 3 core books and at least the Forgotten Realms Campaign Setting. Access to the other supplementary books published for Forgotten Realms may also prove to be handy, if not necessary.
Unlike other previous books that described a portion of Faerun and presented the reader with a huge load of data on the region, as well as prestige classes, spells and magic item connected to that region, Mysteries of the Moonsea takes a different approach when it describes only mayor locations, NPCs and the most important adventuring info on the region and uses the remaining space to present a huge amount of small adventures, each connected to one of the four mayor cities of the region (or to their surroundings). Some adventures are not really more than side-treks or encounters, but most are perfectly suited for one session games. The adventures are loosely connected, and if the DM takes the - considerable - time and effort, they can be chained together to form a campaign from 1st to 18th level, during which the party travels around the Moonsea and makes allies and enemies in mostly all parts of the region.
Unlike other books in the series, there are no new spells, no new prestige classes, monsters or magic items, which may not necessarily be a problem, as there are already a huge munber of these in other books to find the one you really want.
To sum it up, I found the book to be great, it contains enough info on the region to help the DM and the player to get a feel of the people of the region, and most of the short adventures are interesting and have neatly presented maps. Maybe instead of a whole horde of small adventures, a bit smaller horde and one larger adventure connected to each of the 4 cities would have been better, but that may only be my oppinion.
As usual with Forgotten Realms books, the book is beautifully illustrated and edited, but there are certain disturbing errors (like there is no scale on the map).