I don't know if this counts as ebook use per se, but I am a teacher and have a lot of my teacher guides scanned to PDF and saved on my iPad. My storage space is limited and many of the books are on a high-up shelf I can't easily access. When I need to reference them, it's easy to load them into iBooks from Dropbox.
I also keep all my teacher-created resources in Dropbox for the same purpose. I often will load in the student workbook pages so that I can take a screenshot of them. I can import these images into an app called Jot, where I can write on top of them and so model the work for my students if I am showing them a new kind of activity.
I also have some storybooks I wrote for my kindergarten classes. Normally, I print them out and make them into 'book' books, but there was one time where the printers were all down and I had planned to start the new story, so I did actually load it into iBooks and read it to the kids using the iPad.