* The German General Staff (1922): This book is considered a classic work on the organization and operation of the German General Staff, which was renowned for its efficiency and professionalism.
* The Political and Military Situation in 1914 (1925): In this work, Nicolai analyzes the causes and course of World War I, arguing that Germany was forced into war by the aggressive policies of its enemies.
* Secrets of the German War Office (1925): This book is a collection of Nicolai's writings on espionage and counter-espionage, including his experiences as head of the German intelligence service during World War I.
* The War Against the West (1935): This work, published posthumously, is a critique of Western military thinking and a call for a more aggressive approach to warfare.
Nicolai's writings were widely influential in both Germany and abroad, and they helped to shape the development of military thinking in the 20th century. However, his work is also controversial, as it is often seen as promoting an aggressive and militaristic view of the world.
It is worth noting that some of Nicolai's writings may be difficult to find due to their age and limited availability. However, some of his works have been reprinted in recent years, and others are available online through archives and research databases.