Filing ebooks into multiple collections within iBooks

Is there a way to cross-reference your electronic books and PDFs into multiple iBook collections? For example, I want to store a Bayesian stats ebook in the following categories: "All Books", "Education", "Mathematics", and "Statistics". Ideally, I'd like the ability to create sub-collections, so that if I create "Statistics" as a sub-folder of "Mathematics", I could have all my stats books also show up as math books by default, but I'll take just the ability to save a book to multiple collections simultaneously.

This doesn't seem to be possible (or at least I can't figure it out) and it seems like a ridiculous oversight for a digital format that I think I must be overlooking something.