"Even Einstein couldn't get far if 300 treatises of higher physics were published each year"
Raymond Chandler, 1945
Selling books is hard. The best sales come from word-of-mouth, but how do you get critical volume so word-of-mouth does any good? Whether you're published by the traditional press or you self-publish you need to hawk your own goods. Promotional budgets for mid-list authors barely buy a local NPR ad to promote your book signing.
Worse, most writers don't like promotional hoopla and would rather retreat to their favorite quiet place to write their next book. My recommendation: allocate a specific amount of time each day to promote previous books and reward yourself with the remainder of the day for writing your next opus. Personally, I try for between one and two hours a day to handle email, update my blog and websites, and promote my books.