Like other one-party states, National Socialist Germany maintained a formal distinction between "the Party" and "the State", with separate organizations and hierarchies. Thus, what was "official" for the National Socialist German Workers Party was not, strictly speaking, "official" for the German Reich.
In this case, the book was published by Stürmer Verlag, a press run by the National Socialist German Workers Party. Thus, it can fairly be described as "an official publication of the National Socialist German Workers Party."