The best way to promote a book depends entirely on the book and its audience. That’s two things: (1) book subject and presentation, and (2) the intended audience.
There is no one answer for all books. I wish there was just one way, but there are a 1001 ways. Practically speaking, there are about 10 key ways, some more right for one type of book or audience, others more right for another type of book or audience.
If I were to pick one way only, it’d be speaking. It’s a powerful way to introduce people to your books and ideas.
When people here you speak, they become bigger fans. Fans sell books.