(MONEY Magazine) – In 1994, six months after graduating from UC-Davis Veterinary School, Sophia Yin, 36, had an idea for a pocket-size reference book for students and veterinarians, with common drug dosages, anatomy diagrams and the like--"the kind of thing I wish I'd had in vet school," says Yin. "And since I knew I took really good notes, I thought, Who better to do it?"

With roughly $10,000 that her parents had given her for graduation, she self-published The Small Animal Veterinary Nerdbook, spending $7,600 to print the first 1,000 copies and $1,500 to print 3,000 more. Most marketing was word of mouth. Yin then settled into a vet practice, expecting to spend the next five years "working 60- to 80-hour weeks for a salary of $30,000." Instead, she sold 4,500 copies the first year--even though only 2,200 vet students graduated that year--and grossed $102,000. "I immediately enrolled in a personal-finance class," says Yin.

The success of the Nerdbook let her work part time and study animal behavior--taking classes, volunteering, traveling and conducting a three-month study on dog barking as communication. Three years ago, she used her Nerdbook proceeds to fund a master's of science in animal behavior, and she now lectures across the country on small animal behavior. Notes Yin: "There's no way I would have been able to explore what's become my passion if it weren't for that book." --AMY WILSON