If there is a book, it would show up in the mathworks books page. I would suggest clicking the Explore all new titles.
Unfortunately SimMechancs 2nd gen is still very new so it would be tough to find a book just yet. I would highly recomend an alternative to book learning by viewing some simscape webinars
In particular, as you requested, there are simmechanics webinars as well that feature 2nd gen technology. If you're just getting started multibody simulation webinar is a great start.
These are all free and, in my opinion, are an easier way to learn the products since it is all demo-based.
