While Kindle seems to be leading the world in ebook reading, ibooks are adding a million users per week. I do both, but lately ibooks has been leading the race because I like that ibooks handles video within the ebook. Kindles, as far as I have tested, struggle with video. Nonfiction, instructional guides which are part of what I do benefit from including video snips. So ibooks, at least for non-fiction, are pulling ahead of Kindle due to their ability to handle video.