[Adoby]

Another possibility is that the book contains a lot of very big images. Or is made up of just scanned images.

Before you start experimenting, make sure you have a backup of your book and library.

One way to handle this is to scale down the images. Easiest may be to change the output profile to something smaller than "tablet" (Preferences / Common Options / Page Setup) and convert the book from epub to epub.

The output profile "tablet" means that no scaling of images is done. Otherwise the images are scaled to suit the output profile selected. (Don't forget to change back to "tablet" or whatever you prefer when you are done.)

I have not tested this, perhaps you need to save the book to disk for the scaling to happen.

Another way is to simply open the epub and edit the contents. Scale images and remove fonts or videos and so on. And recompress as an epub. But this may be more complicated than the first method.

If the book is made up of a lot of scanned images then you should try to find another version where the images with text has been converted to actual text.