Susan Cooper has published more than 30 books; she’s also written plays, screenplays, poems, lyrics, and short stories. Though she has been a full-time writer for more than 50 years, she hasn’t had two new books out within a month of each other, as she will this fall.

“It’s great fun,” Cooper says from her home on the Massachusetts coast, 30 miles south of Boston. “I’m not usually this busy. But more than that, these two books also represent the two very different sides of my writing life: one half being serious fantasy, the other being things that are just plain fun.”

The Word Pirates (Holiday House/Porter, out now), illustrated by Steven Kellogg, is the fun one; it’s a raucous tribute to the prolific New Zealand children’s author Margaret Mahy, who died in 2012. In it, a band of pirates led by Captain Rottingbones nourish themselves on words pilfered directly from books, destroying tales and making them unreadable.

The blackguards meet their match when Rottingbones takes on the Word Wizard, whom the book describes as “a zany New Zealander whose stories were said to be so wonderful that he knew her words would be extra delicious.” Like the real Mahy, the Word Wizard “lives on a green headland with her cats and her bouncy poodle, Baxter,” and dons a rainbow-hued wig to read to children at story time.

Cooper and Kellogg both knew Mahy from serving on the board of the National Children’s Book and Literacy Alliance. “We’d see each other now and again,” Cooper says, “and [after Mahy’s death] Steven and I were talking about her—Steven had illustrated some books with her, and I’d known her for ages—and we came to the conclusion that we ought to do a book together for Margaret.”

The collaboration was something new for Cooper, who had written the text for many picture books but never in such close concert with the illustrator. “My beloved editor, [the late] Margaret McElderry, forbade me from having any contact with the illustrator—you got the text as right as you could and then it went off to the artist,” Cooper says. “That was fine, but this was fun. And Steven’s pirates are wonderful. It was when I first saw his exuberant sketches of them that the story really took off.”

Cooper adds that she and Kellogg started referring to each other as shipmates and addressing their editor, Neal Porter, as Cap’n. “We even have a picture of Neal wearing a pirate hat,” she says.

Cooper’s other new book will perhaps bring to many readers’ minds her most celebrated work, 1973’s The Dark Is Rising, which won a Newbery Honor in 1974. Like that novel, which centers on the day Will Stanton turns 11 and finds out he can do magic, The Shortest Day (Candlewick, Oct.), illustrated by Carson Ellis, is set on the darkest day of the year—the winter solstice. Cooper says that the seeds for the text were sewn more than 40 years ago, when she attended a performance of the Revels, a “deliberately not religious” celebration of the solstice featuring song, dance, and poetry. After the show, she was introduced to singer and director Jack Langstaff, creator of the Revels, who was a fan of Cooper’s books. “You should be writing for the Revels!” she recalls him telling her.

For the next two decades, Cooper did, contributing lyrics, short plays, and poems. “Whatever Jack happened to want, wherever he had a hole in the program, I would try to fill it,” she says. “He wanted a verse, so I wrote this poem. He liked it, it went into the program, and it stayed there ever since.” Cooper’s chapter book, The Magician’s Boy, is adapted from a play that she wrote for Langstaff’s show, and The Shortest Day is adapted from a poem that she composed for it.

Cooper says her daughter, Kate Glennon, who serves as her manager, suggested that the poem would also work as a picture book. Liz Bicknell, Candlewick’s executive editorial director and associate publisher, agreed.

“I first read Susan’s poem on Dec. 17, 2014, a few days before that year’s shortest day,” Bicknell says. “I was already looking forward to the equinox, when light and optimism return, so to be connected to generations of humans feeling the same way, hoping for peace, this year and every year, was a magical moment.”

Bicknell says that at the time she acquired the manuscript, she was working with Caldecott Honor artist Carson Ellis on another book. She asked Ellis if she’d be willing to illustrate Cooper’s poem. “Carson was already a fan of Susan’s work, and she was excited to do it,” she recalls.

Cooper says she has not yet met Ellis, who lives in Portland, Ore., but is thrilled with the way she visualized the poem, using a muted palette to depict how humans have interacted with—and depended upon—the sun since the beginning of time. “I was blown away by the narrative that her paintings gave to the poem,” she adds. “How lucky can you get, to work with two very different but equally amazing artists?”

Cooper says she will have some choices to make after the busyness of two books in one month subsides. She has a novel written; she has adjudged it is no good.

“Have you ever written a book and then decided it just didn’t work?” she asks. “Mine is titled Soldiers. I wrote it, and then I decided to give up on the whole thing.” The admission should give solace to every other writer with a novel or two in a drawer. “Now,” she adds, “I’m trying to decide between whether I should work on an idea I have for a fantasy or whether I should do another picture book. I’m between things, and it’s not a comfortable spot.”

Not that Cooper doesn’t have plenty to keep her busy. Widowed since 2003, when her second husband (and frequent collaborator), actor Hume Cronyn, passed away, she moved to Massachusetts from Connecticut—to a home in a salt marsh where she has a view of the Atlantic and, she says, can imagine seeing England, where she was born in 1935. She has two children, four grandchildren, and six step-grandkids.

But despite her long career, her list of honors and accomplishments, and a bookcase full of volumes with the name Cooper on the spine, at 84, the British expat still yearns to be productive, to keep finding and telling stories.

“If you don’t write for at least a couple of hours every day, you’re not writing, and the proof to me is that I’m not diving at the desk when I get up in the morning—I’m dithering,” Cooper says. “I long to be back at the business of just working. You finish breakfast and you go to the desk and you stay there until you’re so hungry you absolutely must stop to have lunch.”