Readability versus legibility: in reading, the two overlap but there is a difference. Legibility means all the letters and punctuation are readily recognized. Readability means the eye moves along quickly, with understanding of the text. Every book should be, to some degree, readable.
And if it is, it will also be somewhat legible. However, when readability is the priority, the eye quickly scans over the letter forms, the punctuation marks, sensing them more than consciously recognizing them. That is great for a good reader.
For a beginner, legible typefaces such as Century Schoolbook are preferable. Reading is slower but comprehension, ideally, is improved. When I am writing a draft, I have that same need for legibility. It helps me see mistakes, lets me make sure the punctuation is proper. With some typefaces, it is, for example, difficult to determine which direction quotation marks might slant, or to make out the difference between a comma and a period. That is not a good thing at the drafting stage, nor in early rounds of editing.
I sometimes write with that aforementioned Century Schoolbook. Other times, I prefer to work with a monospaced font at that stage; Linux Libertine Mono is the go-to at the moment, but others work. I do prefer a mono typeface that looks more like the serif fonts one would find in a book. Again, big, easily-read punctuation is particularly desirable.
Incidentally, though Century Schoolbook is great for writing or for children’s books, I would not be inclined to set an adult book in it. If one desires its look, there are ‘tighter’ takes on the Century typeface — more readable ones — or other type designs in the Scotch Roman style that should be preferable.
