Can an author do book promotion on B1/B2 visa in USA?

To me it sounds more like "Business" (which is allowed) than "Work" - if no-one is paying you by the hour to sit in the bookstore, or paying you an "appearance fee" or "honorarium". You're doing an activity that you hope will trigger revenue to some entity, but you're not collecting a salary.

A US government blog expands on the difference between work and business, and specifically lists permitted business activities:

While on a B1/B2 (business/tourist) visa, you may engage in some business activities, including such things as a need to consult with business associates, negotiate a contract, buy goods or materials, settle an estate, appear in a court trial, or participate in business or professional conventions or conferences. You may also use a B1 visa when you will be traveling to the United States on behalf of a foreign employer for training or meetings.

[Bolding is mine.]

However, you may not receive payment (except for incidental expenses) from a United States source while on a B1/B2 visa. Working at a job in the United States while on a B1/B2 visa is generally prohibited and is not an appropriate use of this visa class.

I have come into the US for no reason other than to visit my publisher, and showed the immigration officer my letter inviting me to the meeting, and I was allowed in. While this is not the same as a book tour, it's an indication that going to meetings is "business", not "work".

Also, I have come in to the US for the primary reason of "attending a conference" and at that conference I have signed books, sat on a panel about writing books, etc. I can hope that I got some royalties from that - I get literally a dollar or two when people buy a copy, and the bookstore people excitedly reported selling 5 or 6 copies on one occasion. But it wasn't work - the bookstore people didn't give me a dollar for each copy I signed. The panel people didn't give me (or anyone) a $500 appearance fee. Etc.