You may not have kept it flat between the book or books for long enough. A curled piece of paper takes a "set" after awhile. You need to reset the paper. If it was a single stamp or single piece of paper, you could roll the paper in the other direction to reset the curl and flatten the paper. You probably don't want to do that to a sheet of stamps because you might tear some of the perforations. But flattening the sheet of stamps can be done by pressure if you use enough pressure and enough time. It may take weeks, and it will require not just "a" book but a lot of books or other weight.
I suggest putting the sheet between pieces of glassine paper to prevent any of the glue from attaching to the books from humidity. Place the glassine covered sheet of stamps carefully under a pile of heavy books and wait at least a week or two. Do this in a room with moderate humidity. The sheet should lose a lot of its curl. If not, most people would just put it into a large stamp mount and hope for the best. Stamps mounted in albums often flatten out, so this might flatten your sheet of stamps over time, as well.
Quote: I suggest putting the sheet between pieces of glassine paper to prevent any of the glue from attaching to the books from humidity.
You can also employ "Freezer Bags" (flat) if the wife has any at home. Gum Arabic will not adhere at all to Freezer bags. Place sheet on top of 1 freezer bag, and a clean sheet of paper on top then into the book.