A Comprehensive d20 Sourcebook for Fantasy Role-Playing Games
From the introduction:
One of the most intriguing and versatile character classes in the d20 fantasy roleplaying game system is, without a doubt, the Expert, an NPC class introduced in the 3rd edition of the game. As noted in the description in Dungeon Master's Guide (D&D 3.5e), such characters can include almost any sort of highly skilled professional, including Blacksmiths, Barristers, Merchants, Guides, and Sages. All of these—plus nearly two dozen others, organized into five broad metaclasses—are presented in this book.
A greatly expanded version of the previously published Experts, this item is completely revised for the 3.5 system. Forward by Gygax.