As with any plan (be it a business or the promotion of a book) the earlier you can start the better, you should be planning to start your promotional campaign from the moment you put pen to paper. Now this can of course can be easier said than done (especially if your using a working title which may alter over the course of writing the book), but the theme will remain the same and this still gives you great scope for making the public aware.
Using social media is now one of the most popular ways to get the message of your book out, from the beginning you should be able to make posts which hint at your new masterpiece and spark interest, however, this is a balancing act at times, too many posts turn off your audience (you become viewed as nothing more than a spammer), so keep it to several per day across your channels.
As your book gets closer to completion you should be planning the cover and beginning to increase your promotional campaign, any great book cover designer will able to work with you in preparing your design ready for its edit and possible page count alterations, they should also be able to help in your promotional materials such as banners, posters and 3D product images.
Once you have your book cover designed you should start to use this within your promotional posts and online interactions (as well as off line ones), it may be several months before your book is live but you’re still looking at getting the message across on a consistent basis.
Having product images of your book in several guises helps to make it feel more tangible to your viewer, images of the book as a paperback, hardback and even upon the screen of a tablet will help to showcase your work as a professional publication. You can then incorporate these images into posters, business cards, banners and your website or blog.
Advertising your book early, professionally and consistently will ensure that more people see your work and that it is taken seriously, more importantly, a well thought out approach to the promotion of your book will ensure success and sales.