A site called Similar Web makes it easy for to find sites related to your genre. Let's say you've written an ebook about vampires and want to market/introduce/mention/or whatever your work on sites about vampires. You could google 'vampire books' 'vampire discussion' or 'vampire forum' and go through several thousand listings.
Or you could take the web sites of the top three returned on your search and use SimilarWeb. You input the web address of a site and it returns similar sites, and lots of data to help you make a decision on whether or not to follow-up on a particular site for marketing purposes.
David, some newer sites won't rank as high as well-established sites. This is why statistics like those from Similar Sites should be taken with a grain of salt.
Before Twitter became well-known it would have ranked pretty low in the stats.
For instance, I'm currently working on a new ebook store for authors called Ebook Orchard. It won't have any ranking worth a darn for probably 2 years on Similar Web simply because it is new.
Also, Ebook Orchard doesn't take a royalty fee from authors when an ebook or audio book or podcast is purchased. This fact won't show up in any data from Similar Sites, but it could be important for an author wishing to expand their marketing and promotion range.