Izzy's able to read these text without her glasses because one eye can see clearly enough on its own. Izzy's right eye is a prosthetic. Not top of the line, and it doesn't even have perfect vision. But she hasn't had any real problems with it for the most part. Both of her lenses in her glasses are prescription though.
If you're wondering just how many prosthetics Izzy has that's a question that'll be answered later in the book.