MerricB said:
I'm mostly looking for books that expand the capabilities of existing classes, not invent new ones.
On that note I'll throw my own hat in the ring.
The Heroes of High Favor books are designed around the "favored class" of the various races. Although there's material in each book that's specific to the race, most of the design work is intended to be useful to adherents of the class, regardless of race.
In addition to 10 multiclass prestige classes (the favored class mixed with every other class) there are new Feats of particular use to each class. Tucked in between the Feats and the Prestige classes, about 1/3 of each book, are new rules specific to each class:
DWARVES (Fighters)
Craft Masterwork Weapons and Armor
Fighting Styles
HALF ORCS (Barbarians)
Crossbreeding Animals and Beasts (Handle Animal Skill)
Craft Savage Weapons/Armor
Craft "Shoddy" Weapons/Armor
Initimidate in Combat
ELVES (Wizards)
Customizable Specialist Wizards
Spell Wards
Metamagic by Spell School
Ley Lines/Power Nexuses
Spell Design Templates
HALFLINGS
Pick Pocket Knacks
Dirty Fighting
New Bluffs and Feints
Craft (lashworking)
Craft (ropemaking)
Craft (trapmaking)
Here's a link to ENworld reviews:
http://www.enworld.org/reviews/index.php?sub=yes&where=publist&which=Bad+Axe+Games
EDIT: I should mention, in most cases we've uploaded the entirety of these class-specific rules as previews. Feel free to grab em for free from our website.