It is best to start marketing the book about a year before the book is published.
Offer free talks to groups about it. If it is a book about e.g public speaking, you probably want to go to public speaking conferences like Toastmasters, where you are sure to have an entire audience that is interested in what you are publishing, and will get the community talking about it.
Get on social media and ask for reviews by giving out the advance review copy version of the book (free).
Go to conferences related to the subject matter of the book, and start building contact lists of people who will be notified when you launch the book. Keep in touch with the contacts, build momentum with them. They will likely buy the book at launch time.
These are a few good places to start.