If we asked: How do we sell a lot of books? What is the best way?


Simple answer: advertise.
A friend has done very well with Facebook Ads, but I never have. I've always lost money. I tried advertising on BookBub's pay-per-click ads, got no clicks and so the ad cost me nothing.
Some advertise through email advertisers like Ereader News Today. I did that for a few years, but my 'market' there ended up saturated so the ads stopped paying off. But if you only have one or two books, this is the cheapest way to advertise that I'm aware of.
I'm currently spending a considerable amount of money on Amazon Ads. They're working for me because I have three series and a pair of stand-alone history novels, all on Kindle Unlimited. Sales don't break even, but when the KU borrows are considered, the ads work. If someone buys or borrows the first book in a series, I hope that they'll buy the others and that appears to be happening.
But it's work, so don't expect to put up an ad and get rich. I check mine daily and tweak them every few weeks, trying to get the most return on my investment.
About pay-per-click ads: if you're getting almost no clicks, rewrite the ad. If you're getting clicks but almost no buys, rewrite the book's blurb.