On a spreadsheet inside a bookstore within Porter Square, Kate Mikell tracks disappearances. Concealed in brown paper, a handful of free books hide among the regular merchandise at Porter Square Books, awaiting prying eyes and hands.

Mikell, a bookstore employee, records their eventual absences, noting which shelves need replenishing. She’s been hiding the books—extra advanced reader copies or donations from other staff—for more than a year. It’s become therapeutic, Mikell says.

“It kinda took on a life of its own. Originally, they were just wrapped in paper, and it was sort of like a mystery book idea. And then I started drawing on them,” she explains.

Along with Mikell’s designs, their covers bear greetings and inspirational quotes but omit titles or authors, keeping each book’s identity a mystery. While the books are open to everyone, Mikell hopes they reach shoppers struggling with mental health challenges. She lives with depression and believes books provide an opportunity to escape from “general terribleness.”

“I think with depression you feel isolated,” she says. “And books for me have always been something where you can feel like you’re part of this other world. … It’s something you can do from the privacy of your room, but you can still feel a part of humanity and connected.”

A book disappears roughly once each week, Mikell says. And even though she writes “free” all over them, customers almost always bring their finds to the store’s front register.

“It’s funny because I write, ‘This book is free!’ on them two or three times, and they come up to the counter, and they’re like, ‘Is this book free?’ And I’m like, ‘Yes! Go ahead and take it,’” Mikell says. “There was a girl two weeks ago who found one and just started jumping up and down really excitedly, and it was very cute. I was very happy to see that.”

When she’s not sketching temporary book covers, Mikell runs a secret poetry Instagram and works on her novel manuscript. Over the past year, the mystery books have given her a chance to express her creative side at work and she hopes the people who find them benefit as much as she has.

“It’s always nice to know, when you are feeling down, that someone out there is feeling similarly,” Mikell says. “Something like finding a note or a free book, maybe, can make your day a lot better.”