It varies with the device due to size and weight.
With my K3, I almost invariably use it handheld and naked (the K3, not necessarily me). I'm left handed so I hold it in my right hand between thumb and finders, which leaves my left hand free for important stuff (coffee, eating dinner, coffee, popcorn, coffee, ...). The K3 is light and easy to hold for extended reading. Adding a cover would only add bulk and weight (both negatives). I have a zippered slip case for storage and transport.
With my Win 8 tablet (8" Dell Venue 8 Pro) I find hand holding somewhat uncomfortable due to its weight. Sometimes, like in a doctor's waiting room) I'll handhold it vertically, but most often I use it horizontally with the tablet sitting on a table or pillow arrangement when in bed. I have a cover which folds back to form an easel stand when horizontal. When used that way I do so with the book displayed in two columns. The folded back cover also functions as a grip when handholding is necessary.
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