Context is everything. If you're asking an author to recommend a book on a particular topic, it should be perfectly acceptable for them to mention their own book. If you're asking an author to recommend the best book on a particular topic, they should suggest another author's book or decline due to a conflict of interest.

Of course not mentioning that they are the author, or concealing that they are the author are dishonest and I would see that as unacceptable.