I'm fairly new to developing and want to code an ereader. Could someone point me in the right direction as far as reading the different ereader file types.
Most e-readers use a prorietary format. There are, however some open source formats out there.
Here is a list of all of the popular ebook formats out there (both DRM-restricted and open source): Comparison of e-book formats - Wikipedia, the free encyclopedia
It might also be beneficial to create your own XML-based format so that you can get right to writing your app. Otherwise, you will need to look into the file structore (internal byte representation) of the various formats you want to support. Catering towards all of the available formats (along with your own format) would be ideal, however this would ultimately slow down dev time. It's a decision you'll need to make on your own, as a developer, but keep that in mind.