Hi,
Image compression used to work like a charm for me, and especially, when it had been performed with 0% result, the Save icon remained grayed and you were not prompted to save the book when closing the editor.
But since some bug I did not notice was fixed, you are prompted that the images have been compressed from x% to x% (0%), the Save Icon is activated and you cannot close the editor without accepting to lose the 'changes'...
