Email marketing is one of the best ways (if not the best) to promote your personal brand as an author/writer and your book. People open and read their emails as long as the message is relevant, valuable and interesting. 

If you manage to create a good message to your target audience, to your niche you will get good engagement rates as also confirmed by statistical data (just Google it you will see)

But it may not be easy as a good email campaign has many moving parts: subject line, message itself, links, images, signature, email marketing service provider quality and many others. If you fail to check all the boxes while creating an email campaign all your efforts may go waste.

You may even need to create an email list of relevant, current, targeted people in advance if you don’t already have a list.