I haven't seen this discussed in awhile, so I thought I would throw out the solution without the question being asked:

Here's a little trick I learned to help with damaged paperbacks.  (Don't try this on hardbacks; they are constructed differently.)

For paperbacks that aren't too far gone, a cleaved spine or loose pages can almost always be repaired.  I like to do it at the first hint of a problem; if I see a page coming loose at one corner, I use the repair procedure. 

For almost all paperbacks (some from the first half of the 20th century may use a different type of glue), the glue used to hold the pages to the spine is hot-melt glue.  If the type of damage discussed is in a paperback book, it can often be repaired by carefully using a household iron to reheat the glue.

REPAIR procedure:   Use a heat setting just high enough to melt the glue, usually in the wool to cotton range, and a few seconds will heat the glue without damaging the cover unless it has been taped (such as a former library book).  You may have to experiment a bit as the temperature is not consistent from one iron to another.  Carefully place any loose pages in position (make sure that they are in the right order!) and tightly hold the book together when heating.  You MUST put something between the book cover and the iron--the best is a Teflon or other non-stick sheet, but a plain sheet of paper will often work.  (If you don't, you will melt the book cover coating or ink onto the iron --and that's a MESS!)  Hold the iron tightly against the spine for something like 10-30 seconds.  As soon as you see the glue ooze out the end of the spine, remove the iron.  If you hear a sizzling noise, remove the iron.  Hold the book in position until it cools. 

It is best to do this repair before the spine is damaged by too much bending, but if it's already past that, what do you have to lose?  I take no responsibility if this doesn't work for you, so use it only on books that are damaged already. 

If the glue doesn't melt, you don't have the temperature high enough, you didn't hold the book to the iron long enough, or it's a really old book and uses a type of glue that won't remelt. 

BTW, this is similar to the method that my local library uses to repair their books. 

Thanks for the tip!