Inspect your book to see what type of damage has occurred to the binding. Either the pages have come loose from the spine as a whole unit, called a text block or single pages have come loose, while others are still attached.
If the whole text block has come loose from the binding, stand the book on its spine so you can look down inside the binding.
Dip a knitting needle in PVA, scraping off excess adhesive before applying it to the book.
Use the knitting needle covered in PVA to coat the text block on the side that should be attached to the binding. You may want to twist the needle as you apply the PVA to the text block to make sure the entire surface gets covered in glue.
Flip the book upside down, still standing it on its spine and use the knitting needle to spread glue to the bottom of the text block if necessary. The top and bottom of the text block should be fully covered in adhesive.
Lay the book flat and line up the text block with the cover, gently pressing the text block back into the cover.
Slide a piece of wax paper between the front and back covers of the book so excess glue doesn't stick to the cover and seal the book shut.
Leave the repaired book lying flat and stack some heavy books on top. Leave the book under the stack overnight or until the glue is dry.
If there are single loose pages or a clump of the text block that needs fixing, spread a thin line of PVA along the inside of the binding where the pages are falling out and press the pages back into the whole text block.
Slide wax paper on both sides of the pages that were glued back in to prevent the excess glue from sealing these pages together.
Set some heavy books on top of the repaired book and let it dry overnight.