None. Most of the reference works that I use are better suited to a database than a book. Now I'd be more than happy to more reference works available on readers as databases, but feel that they wouldn't be very useful as ebooks. (To understand what I'm talking about, think about how you use a dictionary on your ereader. You are using an app that your vendor provided, rather reading than a generic ebook, because it is easier to find what you're looking for.)