For all the criticism LinkedIn receives here, this is the kind of situation where it shines. If you have a well-developed LI network, there's a good chance you can search up somebody at Harper-Collins there, find that they're a 2nd or 3rd degree connection, and get at least a weak sort of "warm intro". Best case, you might get a very powerful warm intro from somebody they know well.

Here's another thing you can do... search LinkedIn for people who previously worked at Harper Collins, and cold email a few of them and say "Hey, I see you used to work at HC, I'm trying to work on a deal with them, could I pick your brain for a few minutes". Quite a few people will likely happily agree to a brief phone call / video chat, to help you out. Talk to a couple of such people and there's a good chance one of them will be able to point you in the right direction, and maybe even give you a warm introduction to the person you need to talk to.