I read this book as part of my Honours research project into the Vatican's diplomacy with Nazi Germany. I was told that John Cornwell's "Hitler's Pope", despite the accolades and the best seller status, was a poor piece of academic work, a thesis which Cornwell himself eventually recanted (See the reference in The Economist, http://www.economist.com/books/displayStory.cfm?story_id=3471137).
Indeed it was so poorly researched that even critics of Pius XII did not take the book seriously.
Zucotti's work, the other hand, is a somewhat more valuable resource, as she has rather detailed references to primary documents in her endnotes. Indeed some have their contents spelled out quite extensively in the body.
However, such referencing, buttressed by her award winning status as a holocaust author, creates a veneer of credibility, a smokescreen behind which Zucotti expresses her obvious contempt for Pius XII. This is largely done through her highly selective use of quotes from the primary sources.
Zucotti commits the Cardinal (no pun intended) sin for all historians, begin with a conclusion, use the documents that prove that conclusion right, and either ignore or dismiss the rest. Such an approach runs right through the book.
Where a quote is used that is or can be construed to be critical of Pius XII, she would quote it to the fullest. Where primary documents mention the opposite (and my research showed there were plenty of them), she automatically dismisses the authors of said documents, many of which were eyewitnesses to the things that Zucotti keeps asserting Pius XII did not do, without any justification whatsoever. She uses absolute pearlers in dismissing those authors, such as the classic "He (the eyewitness) should have known better".
Zucotti also uses an artifically narrow criteria to determine the credibility of certain hypotheses put forward by defenders. She demands that documentary evidence be availabe, otherwise it did not happen. Normally it would be a fair criterion, but in the context of an occupation by the most deadly war-machine in the world, the existence of such documents would have placed the possessor and/or author of those documents, and anyone associated with them, in grave danger.
Does Zucotti accept this? Instead she demands that someone with the intelligence to forsee that decades ahead, someone would question the reputation of Pius XII, and accordingly safekeep any written instructions from him. This retrospective projection is by far, the most unreasonable claim for any Historian to make.
In sum, I would say, use Zucotti for her references (for they are quite good), but never subscribe to her silly dismissals, her retrospection and outrageous thesis. For something more balanced on the critical side of this debate see Guenter Lewy's "The Catholic Church and the Holocaust". For the contra, see Ronald Rychlak's "Hitler, the War and the Pope".