I'd argue "buy my book" posts, especially ones posted by the author, shouldn't make the front page of HN. Especially from YC alum. Is this an ad in disguise?

Agreed. The first half of this post is actually interesting, but the second half quickly transforms into an ad. That disappointed me, because I believe the author has something interesting to say.

I'm not the author. I just found the article, read it and found it interesting enough. I don't know who the guy is or even what he does.