@14mission
Lay wax paper down on the pages to protect the book. Take a small paintbrush. Dip it in water and then in Elmer's glue. Paint along the edge of the section that has come loose and then another line on the book where the two should go together. Put the section back where it goes. With the wax paper still against the pages, carefully close the book, making sure that the glued in section lines up with the edge of the book. Put a heavy thing on the now closed book. Let dry for a day.