Marketing is one of the hardest parts of putting out a book. Depending on your method of earning a passive income, you can market only with the use KDP Select, or you can spend large amounts of money per book to have a
campaign, or you can pick and choose how to market through targeted ads on Adwords and Facebook, for example. For some, marketing can be skipped because the heavy use of market research almost guarantees that people will be finding it thanks to the proper use of keywords and creating a valuable and attractive product. Many blogs will also promote books, and if you have any established platforms (social media, blogs, newsletters, YouTube, etc), these are also extremely helpful in marketing. Any chance to cross promote should be taken.
The true key to great marketing is to monitor your campaigns. Figure out which methods are working and which are just wasting time and money, and learn from that to build even better marketing strategies.