eBook Sync Across Platforms

So I have a MacBook Pro and an iPad. I've been using iBooks with great success, mostly because it allows me to import books I get from other sources as I haven't purchased anything other than a few free books. Anyways, I'm getting a Windows convertible, and I'd like to be able to read on it as well but I'm struggling to find a way to make that happen. I'd like to be able to add the book either on Windows or Mac and have it appear on my iPad, and then ideally it would sync reading progress among devices as well. Is there anything that exists to accomplish that? If so, I haven't been able to find it. Any help is very much appreciated. Also, I don't care about the book format, I have ePub's now but I'll convert them to whatever I need. Thank you!