When brothers Mark and Brad Leithauser needed to market their new book of illustrated poems, Toad to a Nightingale: Drawn-Out Riddles, they had to answer a question: Who’s the audience? “Well, it’s not really for children, and it’s not really for adults,” says Mark, 57. His brother Brad, 54, puts it this way: “This book will appeal to adults with arrested ­development—and pretentious children.”

In truth, the brothers are hoping that the book’s pairing of light verse and whimsical drawings can attract more than the developmentally quirky. In the book, artist Mark lends a fantastical eye, and poet Brad a deft ear, to plants (a dropped watermelon), animals (a giant squid), and minerals (mercury, neon, and gold). It’s a lighthearted yet intellectual duet­, framed by a fanciful exchange between, natch, a toad and a nightingale.

Growing up in Detroit, the brothers didn’t have such a collaborative spirit. “When we were boys, I was writing, and Mark was up in the attic drawing, and there was certainly a period of time where we were making a solitary pilgrimage, each with our own artistic ambitions and drives,” Brad says. Mark’s interest in visual art brought him to the National Gallery of Art, where he serves as senior curator and chief of design; meanwhile, Brad’s poetic leanings led to MacArthur and Guggenheim Fellowships and teaching posts at Mount Holyoke College and Johns Hopkins University. (He currently lives in Amherst, Mass., but is planning a move to Baltimore.)

Mark began creating prints and drawings for Brad’s poems in the ’70s, but their collaboration began in earnest with a companion piece to Toad: 2004’s Lettered Creatures, an alphabetical collection of visual and literary ruminations on wildlife.

Mark initially resisted the project, which his older brother conceived. “Brad came for Thanksgiving, and I told him about my idea for a collaboration on an alphabet book,” Mark recalls. “And he sort of said, ‘No.’ But at the end of Thanksgiving weekend, he was done with A and halfway through B. At that point, he was on a roll.”

The work stokes some friendly sibling rivalry. “Mark’s always joking that I’m coming up with absolutely undrawable subjects,” says Brad. Among the mix of darling and outlandish objects explored in Toad, Brad slipped in a few decidedly banal items. Among them: a smoke detector.

“I accused him of actually making a joke,” says Mark. “I imagined that he sat around one night and had a glass of wine and said, ‘What is the most difficult thing we can think of for Mark to illustrate?’” He adds, “Can you think of anything more boring than a smoke detector? It’s round. And white. And that’s it. But I did it.”

The brothers are planning a third book, Good and Gone: Laments For Lost Things, which brings more Leithauser wit and whimsy to the theme of extinct animals and outdated accouterments, from clipper ships to woolly mammoths. Mark is looking forward to another exercise in sibling bonding, especially now that they can work together more often in person instead of over e-mail. “I can’t do what he does, and he can’t do what I do,” says Mark. “But I think we’re both kindred spirits in the work that we do.”

Mark and Brad Leithauser discuss their work Saturday, Nov. 24, at 1 p.m., at Politics & Prose, 5015 Connecticut Ave., free; (202) 364-1919.