This is a book full of useful primary documents as well as excerpts of diaries, reports and other sources. I found the section on the euthanasia program especially useful in my own research.
Unfortunately this book has major problems that make its use by historians problematic. The authors unfortunately did not decide to use footnotes therefore the reader doesn't know where exactly these things come from and if they were simply taken from other secondary sources, as the authors do in a couple sections acknowledge that the section relies heavily on the secondary source of another author.
In addition to the lack of footnotes, the book does not have an index. Therefore if one is looking for something on Auschwitz, for example, one would have to flip through the book, and particularly the section on the camps, looking. This is a huge problem, especially in such a large book.
The maps in the back of the book are also extremely useful and a great addition to the book.