Some people in my network posted about spritz a startup that recently went out of stealth-mode. They do a pretty cool app for speed reading. See this huffington post article for a quick demo and explanation.
They say they are still in development, so the app is not available for the public.
The app seemed seems pretty neat but also pretty easy to do. I said that and people came back with "they probably do a lot of natural language processing and parsing the sentence to align to the proper character" and similar remarks.
So I reverse engineered it. By which I mean opening the demo gifs in Gimp and counting letters. And, surprise surprise: they just count letters. So the letter they highlight (at least in the demo) is only depending on the letter of the word.
The magic formula is
highlight = 1 + ceil(word_length / 4). They might also be timing the transitions differently, haven't really looked at that, I must admit.
Obviously the real value of an app like that is integration with various ecosystems, browser integration etc...
But if you are just interested in the concept, you can paste your favorite (or maybe rather least favorite given the nature of the app?) e-book into my webpage and give it a shot.
The code is obviously on github and CC0.
I wouldn't try to use it commercially though, without talking to spritz.