It's really, really hard to get people to do this and get them to actually do it. There's a reason there's a whole industry around getting reviews, ranging from the total scams (either scamming the author, or scamming readers) via slightly dishonest, to honest but totally useless, to those few that actually works.

The best way I've found (and I have someone doing it for me) is to contact book bloggers on Twitter and offer free review copies. You need to be careful (Amazon's guidelines allow free copies for books, with caveats, such as not being allowed to require a review, or offer any kind of other incentives -- anyone doing this should read their rules carefully). But even that is time consuming and it takes a long time to start getting results (people are busy; you're not paying them; books take time to read; about a thousand other people are begging them to review their books).

It's important though - sales are highly dependent on reviews, and things like advertising a book very rarely has a positive ROI if you don't get everything lining up (title, cover, enough reviews).

So if you find a reliable reviewer, they're like gold-dust if you write more books.

You might be amazed how hard it is to get people to leave reviews of books even if you give them free copies and they read it and claim to like it.