My usual answer to this problem (if the book is worth it) is to pull off the cover, scrape as much of the old glue away as possible and then reapply the cover with binder's adhesive. This isn't a bad method but if the glue is still hard enough you risk damaging the spine.
I was wondering if it would be possible to soften the glue with heat so you could smooth and reflatten it.
I just experimented with my iron on medium heat with a paper towel protecting the book.
I had to gradually up the heat until the glue softened and then I could mold it with my fingers and smooth it down with my bone folder.
Here was the result:
I think it was reasonably successful.
The problem with learning a technique like this is that you immediately feel guilty about all the past patients you lost that you didn't have to. I'm sure Jonas Salk felt the same way.
Warning: If you try this technique, start out on worthless books until you find the right heat level and I WOULD NOT attempt this on vintage paperbacks with laminated covers (Dell Mapbacks for instance)
UPDATE: Turns out you an use an iron to reaffix the laminate as well (thanks to Gary Lovisi of Paperback Parade for the tip)! For laminate fixing, start at a lower temp that needed for the spine, again use a paper towel for protection, and iron from the middle to the edge of a book (or else you might iron a wrinkle in). I've done this a few times and it can greatly improve the appearance of a book.