Predicting demand for library books is a lot like being a buyer for a bookstore: As much as you think you know about their tastes, readers don’t always behave predictably. You might as well be a pollster.

The Ann Arbor District Library stocked up on Donna Tartt’s The Goldfinch and Harper Lee’s Go Set a Watchman last year, expecting both to be as hot as a newly fashionable restaurant, then had to buy even more copies to “satisfy unusually high hold lists,” according to AADL director Josie Parker. At their peak of popularity, the library had 101 copies of the former and fifty copies of the latter. By mid-May, there were sixteen Goldfinches and seventeen Watchmen available, and no holds on either.

Paula Hawkins’ The Girl on the Train, published last year, and Cynthia D’Aprix Sweeney’s 2016 novel The Nest continue to confound calculations. “We didn’t anticipate that they would be the blockbusters of their season,” says Parker, and the library is still playing catch-up. One day this spring, the online catalog showed 120 requests for fifty-one copies of The Girl on the Train, fifty-nine requests for three copies of that book’s large-print edition, and 225 for four copies of The Nest. To ease the backlog, the library ordered eleven more Nests in April.

Turning a book into a movie can throw in a monkey wrench, too. When last year’s film adaptation of Thomas Hardy’s Far from the Madding Crowd was announced, requests for the book unexpectedly spiked. The library had it, of course, “but Hardy titles tend not to be the most in-demand classics,” Parker says, “so we only had a few copies, which we increased. That was an interesting example of a sleeper title, because it was sleeping on our own shelves!”

Hardy’s moment passed quickly, and four copies currently are slumbering there.