Are there studies on the whether it's better to read ebooks in landscape or portrait mode on typical smartphone screen? "Ebook" here implies that the text fills up most of the page with free reflow (e.g. not a PDF that has the same characters on each line in both modes), many pages read one after another; and the font size remains identical between portrait and landscape.
Screen sizes I care about are of typical smartphones, e.g. around 4.5"-5.5", typically circa 16:9 or 16:10 factor.
The measurements defining "better" can be (at least those I can think of) either reading speed, the content retention, eye tiredness, or mental tiredness.