Updated March 18th, 2020- The Bonnier Publishing group made an across the board decision to stop accepting un-agented submissions. Hot Key closed to submissions as part of that.
Hot Key Books is an imprint of Bonnier Publishing which is part of the much larger international Bonnier a large media group.
Hot Key Books is based in the UK. They publish books for kids and teens between the ages of 8-18. The cover art for these books is exceptional. They also have a great, easy to use website. They even have a map to encourage people to visit them if they are in the UK. I think that their catalog is the best part, and very unique. I encourage you to check it out, in part because it will give you a better idea of what they are looking for.
Their staff list is very impressive, most of the people working their have a great deal of experience. The managing editor previously worked for a number of big name publishers including Penguin and Bloomsbury.
Hot Key Books has some established and respected authors on their list already, including Garth Nix. Hot Key Books has already published a lot of books in a wide variety of genres.
They are open to unsolicited submissions. Submitting is very easy to do. Just email them a synopsis and the full edited manuscript of your book (as an attachment). They do not respond to every submission, only the ones they are interested in perusing. So if you have not heard back from them after six months or so, it is safe to assume they are not interested in your novel.
To learn more visit their website here.