It's generally accepted that reviews help sell books on Amazon. Since that's where I sell virtually all of my books, I'm constantly on the lookout for venues to get reviews. The best place I've found is BookBub. I've put several on the site as free books. They racked up 30,000 to more than 40,000 downloads. That brought a large number of reviews, though subsequent sales didn't bring in much more than the cost of the promotion.
I'm reluctant to go the free route anymore at BookBub, but the price for paid books is pretty steep. Promoting a mystery that costs $1.99, for example, means a fee of $930. They promote it to 930,000 subscribers, but to break even I'd have to sell about 668 books. I have difficulty imagining that. On the other hand, a thriller priced at $1.99 would cost $660 with the promotion going to 820,000 subscribers.That would require 475 sales to break even. It might be worth a try.
Meanwhile, I'm trying something new to get multiple reviews. There's a site called Story Cartel where you pay a small fee and they put up a page for your book. Readers can click on a link and get a free copy in exchange for writing an honest review. It stays up for three weeks, during which Story Cartel promotes it, and the writer is urged to do the same. They say you can expect as many as 100 reviews from the promotion.
I put up Overture to Disaster a few days ago and am working to promote it in such places as Twitter, Facebook, and various listserves. If you'd like to try it free in exchange for an honest review, just click on this link.