How do you fix textbook binding in which a section of pages have fallen out of the book?

if the binding / back is solid, simply glue the back of the section with elmers glue and put it back with the book closed. Put enough glue on the back to do the job but don't overdo it with gobs of glue or it will run out onto the sections on either side. If the binding is loose and 'bows out' when you open the book, squirt the glue down there and swish it around with a model paint brush then close it up (put a little glue on the back of the section if the squirted glue didn't cover much of it) Close the book with a weight and shove it against the wall to press the back of the binding shut if it won't stay closed.