Place the textbook upright on a table or other flat surface. Hold the spine as far away from the contents of the book as possible, and apply glue to the interior of the spine. Knitting needles or chopsticks can be used to apply the glue to the spine.
Lay the book flat on the table, and push the contents firmly against the spine. Open the front cover slightly, and push the endpaper of the book toward the spine with a stiff piece of cardboard. The endpapers are the thicker papers you'll find on the reverse sides of the front and back covers. Do the same with the back cover.
Close the book, and use the piece of cardboard to gently press on the outside edges of the spine where the front and back covers meet the spine.
Put the textbook between two boards that are slightly longer than the book. Place 10 lbs. worth of weight on the top board, being careful to equally distribute the weight along the board. If you don't have weights, you can use a 10 lb. bag of flour or sugar.
Let the textbook dry for several hours, and resume studying with your repaired book.