Creating Characters With Personality, and Character Mentor, both by Tom Bancroft.

Tom's one of the '90s-era Disney feature guys. He's done a whole bunch of other stuff too, too long to list here. Anyway, he wrote these two fantastic books on character design (which are really essentially two volumes of one book), and they go far past the basic 'circles and boxes' stuff. They get into the conceptual place where design philosophy and all the "illusion of life" concepts converge. To quote one reviewer, these are basically the books Christopher Hart wishes he could make.

The reason I'd very strongly recommend this, especially to younger artists, is because I've seen several Default Furry Styles emerge over the years. (And I know I'm just as guilty of this as anyone else, and I'm still working to break out of it.) There are certain visual tropes -- body types, facial proportions, particular angles, poses, expressions, etc -- that many artists in fandom fall back on because they're popular and easy. And there's nothing wrong with that, but challenging your default style can lead to some interesting results.

I realize that when it comes to commissions, artists don't always have the time or motivation to engage in complex character design. Sometimes you just want to make deadline, or follow the model sheet, but I gotta tell you: sometimes it really pays off to break out of the same visual routines, even if it's just for single experimental instances. If you're feeling stagnated with your art and can't figure out why, this might be one thing to consider.