I use Viewport Resizer every single day for responsive website design and front-end development. As web developers, we all have our ways to check viewport size and the responsiveness of our sites - but, this one just fits the way I prefer to work better than anything else.
I love these books. I can't say enough good things about the A Book Apart library of books. I have access to all of them on my bookshelf & each one has proven to be a very useful resource to have within reach.