I read your other comment. It looks like you're talking about iCloud Storage there. Books downloaded from the Apple Books Store do not go into iCloud Storage. B

I read your other comment. It looks like you're talking about iCloud Storage there. Books downloaded from the Apple Books Store do not go into iCloud Storage. Books you've uploaded are stored in iCloud. I just checked my phone, I have 169 books downloaded. This includes many books I haven't looked at for years and years. This includes many books that I bought 3, 4, 5 iPhones ago and are still present, copied from device to device, because Books does not remove store downloads unless asked to.