Is there any way to find changes between two revisions of a book? I want to find how much it differs between Pocket Medicine 6th edition and the previous 5th one in scale of percent of words or something similar.

I've revised the translation of the book for a publisher and now they want me to specify the number of pages that was beyond a mere revision; i.e. those I actually translated from scratch.
There are several out-of-box tools that you can use to check the differences between two or more documents. I typically use the built-in merge feature of Mendeley that gives you a confidence interval when merging two documents. From that you can extrapolate if the documents are pretty similar or extremely different.