We signed up, they ask for donations but don't require it. I don't think we have gotten one for a minute, but you are supposed to get 1 book a month and most of them are based off of the kids age, the first one is always the same, I believe it was Dollys favorite book as a kid.
https://imaginationlibrary.com/
They require a local partner to distribute the books, all other admin overhead they cover
Source: https://support.imaginationlibrary.com/article/238-why-is-this-not-available-in-my-area