Do books need a URL when cited? I'm writing my thesis with Latex and I use Mendeley for the citation.
When I add a book and specify some attributes, like title, author and ISBN, Mendeley automatically adds some missing attributes.
One of those attributes is the URL of a book. Usually, this is a URL from amazon.com or books.google.com.

After generating the document with Latex, the URL also appears in my bibliography. It could look like:       

Peter Smith. A Long Book Title. Pearson Education, 2008, p. 530.
isbn: 0-123-54431-2. doi: 10 . 1336 / 976 - 3 - 434 - 03844 - 5. arXiv:
arXiv : 1011 . 1889v3. url: http://www.amazon.com/gp/product/012345678?ie=UTF8&amp;tag=petersmith-20&amp;linkCode=as2&amp;camp=4321&amp;creative=4567&amp;creativeASIN=087239586.


So the URL is very long and, in my opinion, useless. This is not the "real" URL of the book (not the URL of the author or the publisher) and I don't think, it adds any value to the bibliography.

In Mendeley, when I click on Tools-&gt;Options-&gt;Document Details, I can specify which attributes I want to appear. Sadly, I can't uncheck URL in the document type Book. So I assume, this is important?

Can I just manually remove the URLs of all by books?
Yes you can remove the URL as you have the ISBN number for each book; which is a unique identifier, universally given and searchable. 

Note On The Generated URLs by E-Commerce Websites: Also make sure the on the URL you are using. The URL you posted is a generated one from Amazon that has number of variables for marketing purposes.