I kinda feel the most ethical thing for an author to do is focus on a recommendation for the best book that isn't theirs (e.g. besides my book X, I'd recommend Y for Z reasons).
You can acknowledge that someone else's book is well-written, accurate, and valuable even if you think yours is better, or has a different focus, or a different pedagogical method, or...
The rules for the list don't say that an author can't recommend his/her own book. The rules do say that a recommender must explain why he/she recommends one book over others, which this author did.
