It depends a bit on how much text, whether it will be read onscreen or on paper, and whether there are limitations on how many pages there will be.
The most legible is black on white. It helps for the white to be cream rather than bright white to lessen the possibility of the letters "vibrating".
There are a lot of very readable fonts out there. There's a reason why Times Roman, Arial or Helvetica are ubiquitous -- they were designed to be as universally legible as possible for both print and screen uses. There are plenty of other fonts out there that work well. I like Minion Pro for print -- it's slightly narrower than Times, so over the course of a long document you can save a few pages by using it. I like Caslon Pro too (it's the font The New Yorker uses for text), but it has a slightly smaller x-height, so it might lose a bit of legibility at very small sizes. New Century Schoolbook is also good, and is often used for large print books.
If you prefer a sans-serif for print, then alternates like Calibri or Syntax work well. Onscreen fonts that are often used as alternates to Arial include Tahoma and Verdana. If you want to use a serif font for onscreen use, Georgia works fine and is somewhat easier on the eyes than Times Roman.
Most books use a 10 or 12 pt font size, though it might vary depending on the font (and how many pages they're trying to squeeze the text into). People with average reading difficulties (say, middle-aged readers) would definitely need at least size 12. Large print books are usually set in 16 pt or 18 pt.
That's my :twocents: Or maybe a dollar's worth
(Can you tell that I work as a graphic designer?)
onder: