Librarians and bookworms throughout time are the heroes of Anthony Doerr’s exceedingly busy new novel, “Cloud Cuckoo Land.” Think of it as a triptych love letter to the millions of readers who made his previous novel, the Pulitzer Prize-winning “All the Light We Cannot See,” a phenomenal bestseller.

Once again, Doerr presents young people caught in the fires of war, but his stage this time around is far vaster than the plight of two children during World War II. “Cloud Cuckoo Land” struts across millennia. Wear comfortable shoes and remember to stay hydrated.

This is a big novel of people thinking big thoughts. The earliest action takes place in the mid-15th century when Omeir, an ostracized boy with a cleft palate, is conscripted into the Ottoman army and becomes a reluctant witness to one of history’s most consequential battles. The new sultan is marching on Constantinople with a set of mighty cannons that may allow him to breach the city’s ancient walls. (Spoiler alert: He does.) After cleaning army latrines, Omeir “wonders at the mystery of how one god can manage the thoughts and terrors of so many.”

Meanwhile, as preparation for that military assault grinds on, an orphan named Anna works at an embroidery house inside Constantinople. Like the young oxherd outside the city walls, she considers profound questions, too, like “How do men convince themselves that others must die so they might live?” Desperate to raise money to heal her sickly sister, Anna starts plucking ancient manuscripts from an abandoned priory at the edge of the city and selling them to well-heeled Italian book collectors. They work for a pre-Google nobleman who dreams of erecting “a library to contain every text ever written, a library to last until the end of time, and his books will be free to anyone.”

But Anna keeps one literary treasure for herself: a comic allegory called “Cloud Cuckoo Land,” written more than a thousand years earlier by the Greek writer Antonius Diogenes. Highly fragmented pages of this ancient (fictional) codex are interlaced throughout Doerr’s novel. They describe the ordeals of a shepherd who turns into various animals. Some nights, Anna thinks that reading these tales aloud is the only thing keeping her sister alive. This seems unlikely to me.

Some 500 years later, Zeno, an American soldier from Idaho, is captured during the Korean War. He has only the most anxious understanding of his own sexuality, but in a North Korean prison camp, he befriends a handsome British soldier who used to teach classics in England. “Of all the mad things we humans do, there might be nothing more humbling, or more noble, than trying to translate the dead languages,” Zeno’s friend says. “But in the attempt, in trying to drag something across the river from the murk of history into our time, into our language: that was the best kind of fool’s errand.”

Under his friend’s tutelage, Zeno begins to understand the saving grace of those ancient adventures about Ulysses and Achilles. “Because if it’s told well enough,” he says, “for as long as the story lasts, you get to slip the trap.”

Long after he returns home and retires, Zeno volunteers to direct a group of fifth-graders in a play to be performed in an Idaho public library. The children’s script is drawn from a translation of the now-famous Diogenes manuscript known as “Cloud Cuckoo Land.”

And far off in our apocalyptic future when planet Earth is ruined, we follow 14-year-old Konstance, who’s on an intergalactic spaceship called Argos, zipping along at 5 million miles an hour toward Beta Oph2. The ship is controlled by Sybil, a supercomputer nanny that contains “the collective wisdom of our species.” In deference to that great repository, on Konstance’s birthday, everyone sings the Library Day song. To stay engaged for the rest of her space-bound life, she has a multidirectional treadmill, which sounds even more dangerous than a Peloton, and hundreds of scraps of an ancient story called “Cloud Cuckoo Land.”

Amid all the dramatic battles being fought in this novel — past, present and future — one remains constant: the struggle against literary disintegration. Long before “Don Quixote,” an Italian collector is already lamenting, “Day after day, year after year, time wipes the old books from the world.”

Any one of these stories — except the sci-fi tale, which has a moldy “Twilight Zone” funk — might have made a compelling novel. But Doerr has not only packed them together, he’s put them in a blender and then laid out the bits in a great scramble, as though his own book were a textual puzzle as complicated as the ancient Diogenes codex.

“What really matters,” one of the many insightful children proclaims, “is that the story gets passed on.” Yes, libraries are awesome, and we all love books. But the artificial convolutedness of “Cloud Cuckoo Land” is not enough to confer any additional depth on Doerr’s simple, belabored theme, a theme that thumps through the novel insisting that every character kneel in reverent submission.

What’s worse, julienning these disparate plots saps them of their natural drama, and no amount of grandiose narration can pump that tension back in. The fall of Constantinople inches forward so deliberately you’ll think you’re dragging the sultan’s great cannon along the ground by yourself.

That problem becomes even more acute in the contemporary sections. While Zeno and the children are practicing their theatrical adaptation of “Cloud Cuckoo Land,” an eco-terrorist slips into the library carrying a homemade bomb equipped with a cellphone trigger. It’s a terrifying setup, but the scenes are laboriously sliced almost into individual breaths. Had I known the cellphone number, I would have dialed it myself.

Ron Charles writes about books for The Washington Post and hosts

Cloud Cuckoo Land

By Anthony Doerr

Scribner. 626 pp. $30