I have fixed this sort of thing many times. Unfortunately it is an advanced technique and one I don't recommend without a lot of practice. It is easy to damage the sheen or color of the book fabric and generally make the book cover ripples even worse. The basic idea is to cut the edge of the cloth along the board edge, lift it up and get new adhesive into the area without squeeze out. If it is too wet it will make it worse. Alternately, cut the cover near the bubble or even use a syringe. There are so many ways this can go wrong. Many times it is best to leave it. It indicates that some moisture caused the adhesive to release. I hope this helps clarify the situation.