Omer wanted to make sure we uploaded the OneNote 2013 COM API XML Schema so customers could get it if you are programming against the OneNote 2013 API. You can get it here: OneNoteApplication_2013_xsd. It will download as a text file so please just rename it as an .xsd and then you can view it in your XML viewer of choice.
Developing against OneNote is, "interesting". The API is very, very XML and it doesn't have an object model that you'd expect if you've every coded against Word/Excel/PowerPoint. Meaning the XSD really is the "API" for OneNote.
BTW, if you are looking for a more traditional Object Model for OneNote, I've used this in the past, Creating OneNote 2010 Extensions with the OneNote Object Model -> OneNote Object Model, and it's gotten the job done. It's a code only, but it will help you get started..