TSR has a "Villains Lorebook" on their website here that I think has some good example villains that can be mined for your use. AD&D had a book called the "Complete Book of Villains" that had good advice. A post about it here. Wolfgang Baur has a series about villains that I liked a lot. Click here.
Here's more of the Baur stuff -- Wizards doesn't make it easy to find... Part 2 Part 3 Part 4 Part 5 Part 6 Part 7