If there's a lot of glue, I put a patch of kitchen roll paper on top, and squirt some lighter fluid on it, and then put a plastic bag on top to hold the volatile fluid in place while it works. After a couple of minutes, the glue can usually be wiped off easily.
And it does not seem to swell or damage cardboard like water would, so it can be used on books or paper DVD covers, for example. (What a pity it is to design a beautiful cover, just so the next step in the selling chain can put a brazen sticker on it.)
As well as purpose made solvents like "goof off" and "goo gone", the other one that works on plastics, china and glass is olive oil. Don't use that on your books though.
They have Goo Gone at Amazon UK, with great reviews, so I've ordered a bottle. Thanks.