I don't care too much about the text to speech. In my opinion it's of limited utility. I find that the computer voice is sometimes hard to understand because it uses odd cadences. Also, sometimes the results are silly. I was listening to a mystery novel in the car, and at one point a character says, "and then I called 911." Text to speech rendered it as "and then I called nine hundred and eleven." No one says that!
If one wants to listen to a book, one is much better off with a good audible book performance by a real actor or reader, in my opinion. For folks with vision difficulties but who can still see somewhat, I think would get better mileage out of increasing the font size.