It's not a "Will It" cookbook, but Josh does have a cookbook - The Culinary Bro-down Cookbook. It has a few tweaked knock off recipes and some strange originals as well. That was actually my first introduction to Josh, and definitely has his equal-opportunity-foodie flair.
You can get it (Book of Mythicality) pretty much anywhere like Walmart, Target, Amazon etc. As for Josh, his cook book can be found on Amazon. It isn't Rhett and Link based though.
The Culinary Bro-Down Cookbook https://www.amazon.com/dp/145559542X/ref=cm_sw_r_cp_apa_i_y6StCbJXJ0YJW
Mythical Chef Josh published his cookbook ~~last month~~ August 2017. I bought it and it is worth every penny.
Edit: was wayyyyy off on the date of release. Oops
He has written a cookbook. https://www.amazon.com/Culinary-Bro-Down-Cookbook-Josh-Scherer/dp/145559542X
Semi-related, it looks like Josh published a cookbook in 2017 👀