Are you looking for books on the colonies before or during World War I? Empires at War: 1911-1923 has a chapter by Heather Jones on the German Empire. Absolute Destruction by Isabell Hull discusses Germany's colonial policies in Southwest Africa. I know Germany had colonies in the Pacific but I have no suggestions for those colonies.

Even though I haven't read it, i've heard that it's good; The Kaiser's Holocaust: Germany's Forgotten Genocide and the colonial roots of Nazism by Casper Erichsen and David Olusoga.

Other books could include Thomas Pakenham's The Scramble for Africa and John Rohl's Kaiser Wilhelm II 1859-1941: A Concise Life. If you prefer to read in German, Rohl's books are also available in German.