"Book promo" is a shortened version of "book promotion," which refers to activities or strategies used to advertise and increase awareness about a book, such as events, advertising, social media, or other marketing efforts. Full definition
Ad stacking across multiple book promo sites to sell / give away more books and in the hope that your book might stick on the store sites for longer. (marketingsff.com)
Giving your book away for free, either the first in a series on a permanent basis or a short term free book promo, is another way to get people to read you. (bookmarketingtools.com)
For book promo videos, you'll probably want a more polished look. (ourlittlebooks.com)