Anyway, my solution (thanks NG boards). I have had to do this with Tome of Horrors and multiple FFG books.
Go to your local craft store (Michaels, for example) and pick up a bottle of Allene's Tacky Glue. The basic stuff comes in a gold bottle, and should be $2 or less. And pick up a long thin craft stick or something similar if you don't have it. Allene's is similar to Elmer's glue, but is more flexible when it dries.
Open the book with the spine vertical so the gapping begins. Squeeze a modest amount of the glue so it drips down the inside of the spine, and use the craft stick to spread it evenly. Then lay the book flat in a dry warm place for a few days.
Seems to have done the trick for me.