what is the difference between an unabridged and abridged book?

Unabridged means that it is published in its entirety....abridged means that someone has gone through and taken out words/passages to make the book shorter.

I only know this for audio books but:

An unabridged version of a book means it is reproduced in its entirety; in other words the whole thing as written and published by its author.

Abridged is used to indicate that the original version has been edited, shortened, condensed or otherwise modified.

Hope that helps.