title_body: Are there studies on the optimal amount of rows/columns per page? Are there studies on the optimal amount of rows/columns per page as far as usability ease of reading of large ebooks (e.g. hundred+ pages)?
  
  I can see at least three distinct factors playing into this (may be more):
  
  
  The physical necessity of flipping a page as an answer being a cost
  The raster action of the eye (e.g. moving the focus from end of last line to start of next line) being a cost both physically AND concentration wise.
  Tnd the fact that you may lose concentration either from flipping a page, or having to recall what was on last page, if page flipping is too frequent.
  
  
  The measurements can be (at least those I can think of) either reading speed, the content retention, eye tiredness, or mental tiredness.
upvoted_answer: The Studies reveal that personal choice is defined by each person and varies considerably.  72 characters is the old schools typewriter format. According to Wikipedia this is often maintained out of tradition. 
  
  The Effects of Line Length on Reading Online News(Wichita State University, 2005)
  
  
    Summary: This study examined the effects of line length on reading speed, comprehension, and user satisfaction of online news articles. Twenty college-age students read news articles displayed in 35, 55, 75, or 95 characters per line (cpl) from a computer monitor. Results showed that passages formatted with 95 cpl resulted in faster reading speed. No effects of line length were found for comprehension or satisfaction, however, users indicated a strong preference for either the short or long line lengths. 
  
  
  Besides orientation you must also consider font size in combination with screen size, to calculate characters per line.  But in the end yes there are studies, and they say everyone is different.