Independent authors are faced with a bewildering new world when they attempt to promote their books on snd offline. But there are lots of free or very low cost resources online. The following authors and sources do an excellent job of making sense of the contemporary publishing and promotional landscape, and all are worth reading.
Euan Mitchell's Your Book Publishing Options
How to Be an Author from Fremantle Press
Let's Get Digital by David Gaughran
Ricardo Fayet's How to Market an ebook
Ricardo Fayet’s How to Market a Book — Overperform in a Crowded Market
Amazon Ads Unleashed by Robert J. Ryan
Amazon Selling Services from Helium 10
Author 2.0 Blueprint by Joanna Penn
Multiple resources and useful links from Joanna Penn