When I started reading book I had to regularly refer to the english page for a translation. At the end of the book, I seldom need to check the translation.
I assume that the stories written in the 16th and 17th centuries were re-written to update them into current Spanish, as I would think the older Spanish would be much more difficult to understand. As a result, this made the stories easy to read at the same time I could appreciate the skill of the writers.