Alan Schreck is a fantastic author. I had the priviledge of meeting him when he was in Malta. It is a very fine book with clear explanation. This is a very good book that increments your knowledge as you go through it.
This is a fine book that covers the documents of Vatican II well. Done primarily in a question and answer format, it is easily digestible and a fast read. Certainly recommended for all interested readers, it is particularly good for those who have little familiarity with the documents, or those who might be a bit intimidated by the documents (please don't be). I used this book for a class by Dr. Schreck on Vatican II and it was quite helpful. By reading a document and then reading his section on that document, I was helped in my understanding of the document and retention and recognition of its highlights. Also, additional insights and historical context were enlightening. Recommended for anyone, it is most valuable as a supplement to reading the documents themselves (used compendiums like Abbott's are available for pocket change).
As much has been made of the "spirit" of Vatican II, it is indispensible to read the documents themselves, along with helpful aids like Dr. Schreck's book, to cut through the confusion.
(Dr. Schreck has a recent book "Vatican II: The Crisis and the Promise" that I understand is essentially an updated version of this book, but this does not preclude purchasing "The Catholic Challenge" as an inexpensive used book. It is as fresh today as when it came out.)