Little Free Library by Naomi Kritzer

Little Free Library by Naomi Kritzer (Tor.com, April 8th, 2020)1 begins with Meigan building a “Little Free Library” and mounting it on a post outside her house. She puts her unwanted books in it and leaves instructions to “take a book, return a book”. For a short while things proceed as expected, until one day she notices all the books have gone.
Meigan leaves a note to the person concerned pointing out what the rules are. Then, at the end of the same day, she notices that on this occasion only one book has been taken but, rather than leaving a book in exchange, there is a hand-carved whistle on top of the shelves. This object is the first of a series of (increasingly otherworldly) items that are left in exchange for the Meigan’s books: strangely coloured feathers, a green leaf (in February) that looks like a Maple but isn’t, a “carved stone animal too abstract to identify”, etc.
Simultaneous with this the mysterious borrower starts leaving notes (asking if there is a sequel to The Fellowship of the Ring, apologising for the day they took all the books, etc.), and a correspondence develops between the two.
Then (spoiler), Meigan leaves out a book titled Defending Your Castle:

That book was gone the next day.
And a day later, a tiny, glinting gold coin was left behind, with another letter.

To the librarian,
I do not know what I did to deserve the favor of the Gods, but I am grateful, so grateful, for your kindness to me. I believed our cause to be lost; I believed that I would never have the opportunity to avenge what was done to my family; now, suddenly, I have been gifted with a way forward.
Blessings on you.
It you can bring me more such books, I will leave you every scrap of gold I can find.

The gold coin was a tiny disk, the size of a dime but thinner. There was an image of a bird with spread wings stamped into one side; the other showed either a candelabra or a rib cage, Meigan wasn’t sure. Meigan’s kitchen scale thought the coin weighed four grams, which-if it was actually gold-was over $100 worth of gold. Of course, most gold-colored metal items weren’t actually gold, but … it was noticeably heavy for its tiny size, and when she tried a magnet, it was most definitely not magnetic. In theory she could have bitten it, but she didn’t want to mess up the pictures stamped in.
For the first time, she felt a pang of uncertainty.

The borrower (who appears to live in another world) later reveals that their Queen has been usurped, and that, with Meigan’s help (a series of books on warfare), they are going to attempt to regain her throne.
Meigan subsequently provides a series of useful books and accumulates a supply of gold coins in return—and then her correspondent falls silent, before communicating once more at the end of the story to say their cause is lost. The final object they leave is a wooden box, and a request that she keeps the contents safe:

She opened the box.
Nestled inside the wood was a straw lining—and an egg.
It was large—not enormous like an ostrich egg but it filled the palm of her hand. It was silvery green in color, with markings that looked almost like scales.

The egg is the Queen’s child.
This is a well done and charming piece that crams a lot into its short length, but it was too open-ended for me (although I thought the ending quite clever). I wonder if there will be further stories revealing what happened next.
Overall *** (Good). 2,500 words.

1. This story came top of the Locus Poll for Best Short Story and was a Hugo finalist.