Ahhh, it's really not easy, is it?
I picked
Short Fiction because it's a well known genre. You're right,
Poetry is important and has been added to the list.
Now, when you refer to essays, novellas, plays, it seems to me like a different approach of categorizing e-books. One is to categorize genres according to content; the other is to categorize according to type or style of writing. The difficulty here is that we're trying to mix these two which doesn't seem to work very well.