Books don’t always sell in direct relationship to how much their authors love them. Sometimes that’s for reasons outside of our control (such as the pet care book that came out just before Sept. 11, 2001), but there’s often no reason for it at all.

Two of our books, “bowWOW!: Curiously Compelling Facts, True Tales, and Trivia Even Your Dog Won’t Know” and its feline companion, “meowWOW!” (both from HCI, $14.95, 224 pages), remain our little, almost-forgotten favorites: bright, fun and interesting, with illustrations by Molly Pearce so wonderful that we have them framed in our offices. We loved researching and writing these two books. Some fun facts we found:

• Dogs have been taxed for centuries, but the idea of a tag to signify that a dog was “licensed” seems to date to the late 19th century, when Cincinnati started issuing tags on an annual basis, and other cities and states soon followed suit. Although wooden tags for soldiers were used in the U.S. Civil War to help identify the injured and the dead, it wasn’t until World War I that American soldiers got metal tags as standard issue. The resemblance between the tags of soldiers and of dogs (along with a good dollop of droll military humor) soon had the men calling them “dog tags” – a term that sticks to this day.

• The cat has one up on the lion: Cats purr, but lions cannot. (On the flip side: Lions roar, which cats can’t.) No big cat can get his motor running the way our household kitties can, purring constantly as effortlessly as breathing, both in and out. Tigers can rumble a tiger-sized purr-like sound, but on the exhale only.

• All dogs have pink tongues, with two notable exceptions: the Chow Chow and the Chinese Shar-Pei, both breeds with tongues variously described as “purple,” “black” or “blue-black.” Black spots on tongues are common in many dogs, and are not necessarily an indication that there’s a Chow Chow or Shar-Pei in the gene pool, however.

• Most cats have five toes on their front paws, but only four of them hit the ground. The fifth toe is found on the inside of the front paw. This “dewclaw” is the feline equivalent of our thumb, and it’s used for grasping prey and climbing trees. Any number of toes over the norm (usually an extra one or two, but occasionally as many as three or four) makes a cat polydactyl, which means “many fingers.” Polydactylism is a dominant genetic trait, which means just one polydactyl parent is enough to make a litter of polydactyl kittens.

• Helen Keller, the blind and deaf woman whose triumph over her disabilities made her an international sensation, was the first American to own an Akita dog.

• Cats can hear nearly three times more frequencies than humans can. For you technical types, a cat’s hearing stops at 80 kilohertz, a dog’s at 45 kHz, and a human’s at a pathetic 20 kHz. Because cats can rotate their ears and focus each ear independently, they also can hear well from all directions. A cat can rotate its outer ear to locate a sound – such as the sound of a mouse’s footsteps trying to sneak by – 10 times faster than a dog.

• The phrase “Beware of dog” is so old that its Latin equivalent – cave canem – has been found on signs in Roman ruins. The word “watchdog” isn’t quite as old, but it has been around a long, long time. The first mention of it? By Shakespeare, in “The Tempest.”

• Cats’ heads come in three basic shapes: round, such as on the fluffy Persians; triangular, such as on the sleek, show-bred Siamese and other so-called “Oriental” breeds; and rec-tangular, such as on the burly Maine Coon. Most random-bred cats tend more toward the triangular head, albeit a less extreme version than on the show-quality Siamese.

• One final one, just for summer: The “dog days” of summer have nothing to do with dogs and everything to do with the brightest star in the night sky: Sirius, the constellation also known as the “dog star” that’s highly visible during some of the hottest weeks of the year.