That's quite unclear, yep. He also uses affiliate links to the book on Amazon. Seems shady.
Given that, and seeing as the blog post author was upfront about it, I don’t see a problem with it unless the book’s author objects. Affiliate links don’t even steal credit — they give an author’s work more visibility to people who might be interested in it without cutting into their revenue. A commission is an incentive to share work you enjoy.