If you have any of the Complete... series (Arcane, Divine and Warrior) then you pretty much know the score on this one. It contains additional material to spice up specific character types - in this case, "adventurers" (basically rogues, but broadened out to include "skillful" characters of all classes). And in line with the other books, it contains new core classes, new prestige classes, new feats, new equipment and new spells.
I personally don't really like the new core classes (in any of the books) as most of the concepts seem better for prestige classes (the ninja, scout [a sort of wilderness/skirmisher rogue] and the spellthief [steals powers from spell casters]. The prestige classes are nice, with some designed to benefit from interesting multiclass combos (rogue/wizard, rogue/druid and rogue/paladin). The new feats (some updating previous versions on the previous 3.0 versions of these books like Song & Silence) are nice (I really like Oversize Two-Weapon Fighting, which allows you to treat a 1-handed off-hand weapon as a light weapon - how about a dwarven waraxe in each hand?). The spells section has some nice updates for stuff like the assassin spell list, and the new equipment has some interesting ideas like alchemical pellets mixed with some (to me) slightly duff ones (some complicated exotic weapons that require certain skill levels to get some pretty minor additional benefits - not a bad idea but not nearly well-enough developed).
Together with some interesting material on additional uses for existing skills (a boon to DMs as it deals with quite possible situations where adjudication might not be obvious) and material on how to develop organisations (enemy or ally alike), this is a pretty handy volume. Given the slightly samey format, they don't really inspire as works of art. But this is a good tome which provides a toolbox to players and DMs alike to develop the game.