Seven years ago (yikes!) I published a post on paperback restoration that was picked up by a number of crafting/DIY sites. The post described how to use an iron to gently heat the glue of a paperback binding to reattach loose pages/sections and to resquare a cocked/warped spine. I stand behind the technique (since friction and heat are the forces that deformed the book it makes sense to use heat to restore it) but I have modified it a bit over the years. I also recently acquired a new tool that works much better than a standard clothes iron.
The "new" tool is a vintage laboratory heating plate acquired on ebay for <$40 (but you might easily turn one up for less at a school or business auction)
The heating surface on this one is about 7" diagonal (about the length of a paperback spine), it has finer adjustment than an iron and--because of the horizontal orientation--you can use the weight of the book itself to help resquare the spine. It can also be used to cook up book-binding paste.
My original post on ironing a paperback spine is below (with edits).
I frequently find vintage paperbacks where--due to reading-wear--the glue has separated from the text block and then frozen/dried in a way that prevents the spine from laying flat.
One easy and low-impact way to correct this flaw is to use an iron (or other adjustable heating element) to soften the glue which can then be resquared and allowed to dry in a book press (or under a suitable weight).
I first did this using a paper towel to protect the spine but I have since had better results using a sheet of paper covered with strips of masking tape. The tape backing tends not to adhere to the bookbinding (which can lead to chips and peeling...bigger problems than you are correcting).
The paper/masking tape layer also keeps small beads of hot glue from adhering to your heating element.
Here is the result:
WARNING: Use this technique at your own risk. Results vary with books from different publishers. Start out on valueless books until you find the right heat levels and I would be very cautious (and use much lower heat) on paperbacks with laminated covers.