If you've ever browsed the selection of a well-stocked book store, you know that there is a magazine out there on every subject in the world. And if the magazine exists, so do its readers. You can tap into this existing market to sell your books.
First, you'll want to take a look at your book and narrow down two or three direct genres your book falls into. This covers the shelf your book would sit on in the store along with any sub-genres or ones that are closely related. Write this down.
Second, go through your book scene by scene and identify any subjects that can be realistically tied to your book. This one is a bit harder so here's an example. Your character is out camping in the desert and loses his gear. He's forced to forage for food, build a fire unassisted by modern technology and create a lean to without tools. Now, your book might be a Romance. This is just one random scene out of many. In this case survival skills can be a relational connection. Again, write these down.
Now, go into a store, library or man your computer to search for magazines that focus on your genres or your scene subjects. Gather contact info for the various editors and make a quick note if this magazine publishes book reviews.
For any that will publish a book review, send a request. Make sure you explain in your review request why you think their readers will want to know about your book. Keep in mind, these magazines don't exist to sell books for you. They are only going to publish an article if they feel it is a good match for their readers.
If the magazine doesn't offer reviews, this is where your relational connections come into play. You can pitch these editors ideas for articles that you can easily tie into your novel. Using the example above, you can talk about how to build a structure in under an hour, what survival tools you'll want to always have in your pocket, or edible plants in the dessert. You'll open by referring briefly to your character and then write a solid article.
And just because a magazine doesn't usually do reviews doesn't mean they won't. After the magazine has approved your pitch for an article, thank them and offer to send a copy of your book if they're interested. They may not publish a review in the magazine, but they might be more inclined to include a short promo for the book and/or to put up a review on Amazon.
With magazines, you have the ability to expose a whole new market of readers to your work