While shameless self-promotion would be an issue, I think this is exactly in line with what HN (and even Show HN) is for. Although that's really up to Dang et al.

It's a topic that is highly relevant to this community and for which there is high demand, given the number of Web Assembly tutorials that make it to the front page. Obviously, it's commercial, but so are many of the projects in Show HN. The ones that aren't are still self-promotional in some manner.

I often find that looking at book sites is an interesting way to learn what technologies are getting popular and I'm always curious to see the projects used to teach these things. Posting a book's website can start a conversation, whereas an Amazon link is more likely to start purchases and little more.

Finally, writing a technical book is an impressive accomplishment. Specifically, it's an impressive technical accomplishment.

Regarding price, $29 seems quite reasonable for a technical book. Many cost $35-45, and that's without getting into textbooks.

Show HN is explicitly self-promotional... sure, it's usually used for software projects of some sort, but I don't see any reason that it shouldn't be used for a book you wrote, especially when it's very topical, relative to the community here.