Our community member Markus Stubbig wrote a VyOS book in German that is now available for purchase from Amazon or bob.de.
Markus says there are no definite plans for an English version yet because he's not confident about his translation skills and will need help. If you have those skills and want to offer your services, we can connect you with Markus.
We don't have copies of the book yet (and none of the VyOS maintainers are fluent in German either, though I can read it a little), we cannot provide any kind review of the book yet, but we have no reasons to doubt Markus' expertise. If you get the book and write a review, we may publish it in the blog.