Preface. I've published eight books--4 nonfiction (U.S. history) and 4 novels. Here's what I've learned about the book market since I published my first book in 1975.
Reviews. Getting reviews for nonfiction books and novels is very different. My U.S. history books were widely reviewed in historical journals without much effort on my part. All it took was for my publishers or me to send review copies to the relevant journals in the field. By contrast, reviews are tough to come by for fiction. There's just too much competition: 200,000+ novels published in 2016 alone. I managed to coax a few local papers into reviewing my novels, but that was it.
Sales. All four of my books in U.S. history sold at least 1,000 copies and several sold substantially more. But having read in advance about the market for fiction, I was prepared for the fact that most of those 200,000+ novels published annually sell less than 200 copies. That's about what the three volumes in my Buenaventura Trilogy sold. I don't have the early returns yet for T.T. Mann, Ace Detective (August 2018), but I'll be pleasantly surprised if it sells more than 200 copies.
Publicity. This wasn't a problem for my nonfiction books. My publishers helped me get reviews and they put the books on display at historical conventions attended by thousands of history professors. With novels, at least with those published as mine were by independent presses, the author has to do most of the publicity in competition with an overwhelming number of novels coming into print. Result: Mostly lost in the crowd.
Social Media. Social media is supposed to be the contemporary way to get noticed. Fair enough. I paid a company to send out tens of thousands of tweets and emails over a four-month period. I tracked these e-blitzes and got some fun responses, but few sales. My impression is that Twitter, for example, is a site for sellers (novelists trying to sell their books) not buyers (folks looking to buy someone else's book), or at least that sellers vastly outnumber buyers.