Not all non-fiction has pictures. Many biographical, scientific, historical, and social studies
written for broad public consumption (i.e., not college students) do NOT have pictures.
I have 1491 by Mann in eBook form. Several Barbara Ehrenreich eBooks. The Herman history and sex study books. James Randi and Martin Gardner's skeptic / science eBooks that are available. Quite a few books on religions... etc. These are all non-fiction.
I dunno if 80% is reasonable. (And don't care.) But non-fiction as a whole is not all pictures.