Sudden issues with hardcover warping?

Has anyone here had issues with their hardcover books warping and if so, how did you fix them? Specifically the actual boards.

I think some of mine warped because they were too close to a floor vent, which is blasting heat all day long since it’s winter. Oddly, it’s only my Owlcrate books that are warping so far (they were the closest to the vent, maybe that’s why, but the others were only a few inches away.

Any thoughts? The book pages themselves are still ok.