Here's a description of the book in English (albeit one translated via Google and myself):
In the brief history of cinema, there have been numerous adaptations of world literary classics and, as one would expect, some have been more successful than others. "Don Quixote" is one of those masterpieces that has often found its way into the cinema, either in full or in part. There have been over a hundred films produced that have attempted to adapt the novel, or have used the character of Quixote as a reference model, from a variety of countries including Brazil, Finland, Greece, Argentina, Denmark, Israel, Bulgaria, Australia, Switzerland, South Korea, Italy, France , United Kingdom, United States, the Soviet Union and, of course, Spain. In these pages, Ferran Herranz unravels the most publicized of these film adaptations and addresses the difficult relationship between literature and cinema, which, at this point, has failed to make an adaptation of "Don Quixote" that has generated a unanimously favorable reception.
I have no idea how much of the book is devoted to Welles' adaptation and I don't think there is an English translation. An interesting find nonetheless.