What did children do before Disney? They read Beatrix Potter. They still do. Her Peter Rabbit, who first appeared in 1902, still has a world audience, and royalties from her other books and “licensing kingdom” (as Linda Lear’s publisher puts it) earn something like $500 million a year. The new film about Potter’s life, starring Renée Zellweger and Ewan McGregor, will make that gross even more.

Unlike Disney’s Mickey Mouse & Co., Potter’s Peter & Co. were set in “a real place and in real, rather than imagined, nature,” observes Ms. Lear in her new biography “Beatrix Potter: A Life in Nature” (St. Martin’s Press, 554 pages, $30). This meant, for example, that Potter (an exquisite illustrator) dressed her rabbits in human clothes but pictured them in postures and positions these creatures habitually assume. My favorite illustration is of two rabbits in a potting shed. One is entering the building with a tray of flowers as another tends to a flowerpot. This pen-and-ink study vividly creates a gardening world, “replete with pelargoniums and fuchsias in claypots, with a collection of gardening tools — rakes, hoes, brooms, spades, forks and a large watering can.”

Potter outdoes Disney and other animated cartoonists in her romance of details at the intersection of the animal and human worlds. From childhood on, Potter lived with animals, making pets of mice and rabbits and all sorts of wild things.

Potter did not become a children’s book author until she was nearly 40. But her long apprenticeship as an observer and illustrator of the natural world served her well. She was an amateur scientist who put nature under her microscope and a conservationist who left a tract of land several times larger than New York City’s Central Park.

Born in 1866, Potter belonged to a socially ambitious and wealthy family that expected their daughter to marry upward. When she fell in love with her editor, the family rejected his proposal of marriage, since anyone in publishing was considered no better than “in trade.” Potter thought her parents’ pretensions were ridiculous, since the family fortune had been built on the cotton trade. But there is no snobbery quite like that of the nouveau riche, which places a premium on social climbing.

But what of Potter before 40? Did she have no beaus? Ms. Lear takes her time explaining that Potter was shy and disliked the elaborateness of Victorian courting rituals. And by the 1890s she was too old to behave like one of those “new women” that H.G. Wells and Bernard Shaw had so much fun writing about.

Potter was quite willing to defy her parents when the right man came along, but her editor died a month after the announcement of their engagement. A devastated yet resilient Potter eventually found love again when she married her solicitor, another suitor whom her parents had rejected.

Potter’s attitude toward her family and marriage reminds me of Charlottes Brontë’s. Both women wanted to be good daughters, but they had the independence of mind to seek happiness outside the patriarchal home when it was offered to them. Potter was fortunate in that her only two suitors were sensitive to her genius, and in that other men, too, did their best to promote her talent within the strictures of Victorian society.

In Ms. Lear’s account Potter emerges as a determined woman, yet one who was in no hurry to develop her talent, which began with copying pictures she liked, studying the anatomy of animals, and then adapting her knowledge to “picture-letters” she sent to children.

The photographs, drawings, and watercolors in this biography require considerable study. There is a portrait, for example, of a sheep’s head that is done with such gravity and care that it rivals any presidential portrait I’ve seen. “Her skill impressed her shepherds,” Ms. Lear notes.

Compare the photograph of a beaming Potter holding her Pekinese dogs, Tzusee and Chuleh, to one of her parents adopting dour poses for the camera. That the ebullient Potter could have emerged out of that rigid world to live on in such triumphant old age is surely a great achievement, one this biography superbly commemorates.

Ms. Lear’s ability to meld narrative and analysis is very impressive — so much so that whether you know much or little of Beatrix Potter, you will be enchanted by this story of a supremely gifted and ultimately happy human being. Potter’s story has been told before, of course, and Ms. Lear gives due credit to her predecessors. But this book’s level of detail and acuity makes it as nearly definitive as biography can be.