Most modern books are made using mechanically applied hot-melt plastic glue on the spine. This can be a very effective adhesive but a problem to remove as they are not soluble in most common solvents. The spine can be ploughed or guillotined off, but that may not be appropriate in many cases, especially for sewn bindings. This short video shows how most of the glue can be removed using gentle heat. You may wish to do this if re-binding or repairing a book, though if you are not changing the shape of the spine and the glue is in good condition, then it may not be necessary to remove it at all.