Autumn: when the falling leaves drift by the window, and the days dwindle down to a precious few, and songwriters wax especially melancholic. It makes sense that we celebrate our spookiest holidays, Halloween and the Day of the Dead, when the newly crisp air carries with it premonitions of loss and mortality. But autumn is also a time of renewal and harvest — and of reckoning, as in Judaism’s Days of Awe. It is our subtlest season, our bittersweet season, one that can prompt feelings of deep yearning. C. S. Lewis wrote about this in “Surprised by Joy,” when he recalled reading Beatrix Potter’s “The Tale of Squirrel Nutkin” as a boy: “It troubled me with what I can only describe as the idea of Autumn. It sounds fantastic to say that one can be enamored of a season, but that is something like what happened … the experience was one of intense desire.” For me, fall evokes a kind of present-tense nostalgia — a wallow in fleetingness, perhaps.

I hope “Wee Sister Strange” reaches a large enough audience to trouble an entirely new generation with the idea of autumn. Its eeriness will merit repeat Halloween readings, as you may have guessed from the title, but it also echoes with deeper seasonal resonance. The story begins:

They say there’s a girl

Who lives by the woods

In a crooked old house

With no garden but gloom.

She doesn’t have parents.

No one knows her name.

But the people in town

Call her Wee Sister Strange.

One thing I admire about Holly Grant’s verse is its pluck: Sometimes it rhymes and sometimes it doesn’t; it goes where it wants, but always seems to scan. Wee Sister Strange turns out to be as independent-minded as her creator. She is very much an autumnal creature, an October forest sprite with yellow eyes and a garland of red, orange and yellow leaves in her auburn hair. She cavorts with owls and bears, enjoys a wary détente with wolves and is equally at home in the bog where “she swims oh so deep / And she walks on the slime / Where the bog creatures creep.” K .G. Campbell’s illustrations are both gorgeous and mysterious — again, seasonally appropriate — and he manages to make even that slime and those bog creatures alluring. But he saves his best for first: The book’s initial spreads, before night falls and Sister’s adventures begin, are masterpieces of waning yellow-orange light and lengthening purple-grey shadows.

We eventually discover that Sister is searching for something. What that is, and where and how she finds it, involves a leap into bedtime metafiction that could have felt forced, or cute; instead, the ending strikes emotional chords that are hard to articulate but should be familiar to anyone who has felt the pull of October’s shifting moods, its sorrow and comfort. I love “Wee Sister Strange.” I think it would have sent C. S. Lewis over the moon (a harvest moon, preferably).

“The Call of the Swamp” is more of a November tale. In Marco Somà’s illustrations, the brown leaves have mostly fallen, the skies are cold and gray, and nearly every spread features a light but steady rain. The emotional temperature is also grayer, sadder. Like “Wee Sister Strange,” Davide Cali’s narrative unfolds with the matter-of-fact oddness of a centuries-old fairy tale. It begins, as so many such stories do, with a childless couple. When they find a newborn at the edge of the swamp, “it seemed like a gift from heaven, and they paid no attention to the fact that he had gills like a fish.” They don’t worry if he has parents either, “because he had found a new mom and dad now.” They name him Boris.

Boris. That made me laugh. “The Call of the Swamp” pulls off the rare trick of blending whimsy with genuine ache. Boris has a good life with his parents. He goes to school, rides a bike, plays with friends, is loved. But his three pairs of wavy external gills mark Boris as an outsider, and one day “a salty smell” borne on the wind — “the scent of the swamp” — stirs a longing for old haunts. He runs away … but from home or to home?