I use my tablet (Galaxy Note 10.2) almost only to read eBooks (>90%). I always read in landscape mode if the format is epub. This way I have two pages open in almost the same size as when I read a normal paper pocket book. Of course, technical literature in pdf format I usually read in portrait mode. Whatever works best.

Btw...when I bought my tablet a few years ago, I tested quite a few readers. At that time cool reader was the best I found:
http://coolreader.org/e-index.htm
Maybe in the meantime a better one was released, but I am still happy enough with this one that I feel no pressure to search for
alternatives. One of the best features: It is open source.