Flowers in the Gutter includes endnotes, a bibliography, and an index for reference. However, many of the sources used are either exclusively in German or hard to access. Here are some key texts and websites in English .
Peukert, Detlev. Inside Nazi Germany. The translated version of Peukert's book provides a unique social history of the Third Reich, and how people conformed, didn't conform, or resisted. Peukert also wrote a book about the Edelweiss Pirates, and include some key parts of their story in this book.
NSDOK - Nazi Documentation Center Cologne. Located in the El-De House, the former Gestapo office and prison, the NSDOK is a memorial, museum, and research center. The English version of their website offers a translated tour of the museum exhibits, overviews of their current research projects, and background information on the El-De House.
The United States Holocaust Memorial Museum. The USHMM is much more than a museum. In addition to information about their exhibits, you'll find a reliable encyclopedia and background information on the Nazis, World War II, and the Holocaust. You can also search the collections, which include documents, photos, artifacts, films, books, and more.