Among the folk beliefs that book publishers rely upon in choosing what to publish, few have been more reliable over the years than the theory that the ideal book to have on one's list would be called Lincoln's Doctor's Dog. Books about Lincoln sell, we all know that, and books about (and by) doctors sell, but books about dogs outsell the two combined. Anybody who doubts (or forgets) this piece of wisdom need only look at the number of weeks John Grogan's Marley andMe:Life and Love with the World's Worst Dog has been a #1 bestseller.

But it's not just dogs. Rabbits have racked up enormous sales figures (look at Beatrix Potter, or Richard Adams's Watership Down), whales sell (Moby-Dick), birds sell (Jonathan Livingston Seagull), deer sell (Bambi), horses sell, and sell and sell (Seabiscuit; National Velvet) and cat books outsell almost every other category. Even sharks, though hardly cuddly, sell (Jaws). In fact, there's hardly anything that moves on four feet or flies or swims that hasn't been a bestseller at one time or another. Of course, you can go too far. There is the famous story about Alfred A. Knopf, who once sent a copy of a book on penguins of which he was very proud to a young girl, the daughter of a friend, and got back a thank-you note that read, "Dear Mr. Knopf, Thank you for the lovely book you sent me. It told me more about penguins than I wanted to know."

One of the things that makes writing books about animals easier for authors is that it's easier to write about them than about humans. First of all, they can't sue for libel, or complain, but also, more importantly, complicated as the lives of animals can be, their motives are never as muddled, let alone downright evil, as are those of humans, even in the case of the most highly anthropomorphized animal characters, like those in Watership Down, or Richard Adams's strange little book in which the American Civil War was described through the eyes of General Lee's favorite horse, Traveler. There are bad dogs, as it were, but no evil dogs—even the Great White Shark in Jaws is, after all, only doing what comes naturally. (It may seem evil to us, but there's no way it can seem evil to the shark.) That is why the best writing about animals is done by those who don't try too hard to give them human feelings or motives.

But what people want to read about is animals that do think like humans, or at any rate feel like them. In the wild, that's a dangerous fallacy, as the human central figure of the film Grizzly Man discovered, when he and his unfortunate girl friend were killed and eaten by one of the bears he thought he was protecting (our feelings about bears are molded by Winnie-the-Pooh and Baloo the Bear, but bears haven't read Winnie-the-Pooh or TheJungle Book, alas), and as countless people discover every year when attempting to pet everything from cats to lions, frequently with unhappy results.

When you think about it, a substantial portion of the literature and even the entertainment that we absorb in a lifetime (and the majority of what we absorb as children) comes to us in the form of books in which the characters are animals, particularly in the English-speaking world (though anybody who has been to school in France can probably still recite some of la Fontaine's fables by heart). Our childhoods are populated by hedgehogs, donkeys, bears, toads, the elephants in Babar, the animals in TheJungle Book, otters, snow geese and a whole menagerie whose lives are richly imagined, and whose characters are as firmly established in our minds (perhaps much more firmly, since we come to them younger) than those of Dickens, Jane Austen or Tolstoy. Even our moral guidelines tend to come to us from the mouth (or beak) of beasts—the tortoise and the hare, the industrious ant, the cricket, the spider; the moral fables that we live by (and which stick in our mind) are those that come to us from animals in books, so much so that it is sometimes hard to look at animals in the wild or in a zoo without expecting them to turn and give us a lesson in not being greedy or waiting to the last moment to do our homework.

When we sat down to write Cat People together—for two people who are married to collaborate on a book together for the first time is a dangerous adventure, like going over Niagara Falls in a barrel—we thought we were going to be writing about people we knew who had bizarre relationships with their cats, or whose lives had become totally dominated and defined by cat ownership, and the book is that, to a degree. But in truth, the cats took over the book, rather the way that Traveler took over General Lee's story. We found ourselves writing about the cats—about our cats, mostly—as if we were ghostwriters for felines, telling their story and their feelings in their own voice, or at any rate in the voice that we imagined for them. So much so that they've taken over, and one of us (Margaret) is now writing a children's book about a cat named Kit-Kat and her friendship with a stray bird (a guinea hen), and daily writing cat and bird dialogue as if that were the most natural thing in the world.

In the meantime, our cats sit with us attentively as we write, Margaret upstairs in her office, Michael downstairs in his. Their expressions are mildly curious, but strongly suggestive of the notion that we couldn't be writing about anything else than them. The youngest of them, George, occasionally takes a stroll over the computer keyboard, as if he were impatient with his ghostwriters and determined, as so many would-be celebrity authors are, to tell his own story in his own words. The older ones, like movie stars and sports figures, are content to let the writers scribble away in their name, and will be available when the book finally appears to do publicity and accept praise.

Even the horses, now that we've written about them in Horse People and Horse Housekeeping, look as if they are expecting their stories to be told. As for the guinea hen (known simply as "the bird"), it can't be for nothing that he or she (it's hard to tell with guinea hens) likes to sit at the window of Margaret's office and look in while she's typing.

Actually, when you think about it, why bother with human characters at all?