Publishing has a secret weapon that most other industries don't: passionate readers who promote books for no reason other than to spread the joy of a good story. These readers also seek out recommendations from others in their book community. Strong communities exist for all types of books and authors and new ones are created constantly. They vary in size, location, and membership but all share the love of reading as their core inspiration.
Community endorsement is often the most important part of book promotion, although it may mean different things to different authors. Aside from the perennial bestsellers like Janet Evanovich and James Patterson, there are not many first-time or non-famous authors on the bestseller list. The ones who do make it usually have a community of peers and devoted readers behind them spreading the word about their book.