Every day I get 3 or so questions and most of them are answered by pointing at the correct reference. Buy the books first! Every collector I know uses book author and page to reference questions. Like Stubbs pg 256 for example. The "must have" selections are in bold. These are my top choices and only an opinion.  Others have different opinions. I am terribly behind in reviews. Check back often I am updating. If you are language challenged (I get a lot of this), take a look at Imperial Colours below. It might help.
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Pickelhaube books     Atlas des Deutschen Reichsheeres   Casques a Pointe Vol 1&2 Lacarde   C.E. Junker's Catalog 1932   Das Deutsche Reichsheer   Das Reichsheer und Seine Traditions   Deutsche Helme 1897-1914   Deutsche Officerhelm aus der Kaiserzeit 1870-1918   Die deutsche Infanterie   Die deutsche Kavallerie   Deutschlands Armee in feldrauer Kriegs und Frieden Uniform   Deutschlands Heer und Flotte   Die feldgraue Uniformierung des deutschen Heere -- 2 Volume Kraus   Dress Uniforms of the Imperial German Cavalry -- 3 volumes   The Emperor's Coat   Formations und Uniformierungsgesichte -- Pietsche   Führer durch Heer und Flotte   The German Army in the First World War -- 1 volume Kraus   German Artillery 1914-1918   German Infantry 1914-1918   Head Dress of the Imperial German Army 1880-1918 -- Saunders   Helmets and Headress of the Imperial German Army 1870-1918   Imperial Colours   Imperial German Field Uniforms and Equipment Vol 1,2&3 1907-1918   Imperial German Headgear- Turinetti & O'Connor   Imperial German Officer's Helmets and Headress 1871-1918 -- Stubbs   In the Service of the Kaiser   Jäger und Schützen   The Kaiser's Army in Color   Konigreich Wurtemburg die Militärische Kopfbedeckungen 1869-1919   L' Armee Allemande en 1914 -- Laine   Le Repertoire de L' Armee Allemande   Major Menzel's Dienstunterricht des Deutschen Heer   Militäriche Kopfbedeckungen der Kaiserzeit   The Pickelhaube -- 2 Volumes -- Bowman   Pickelhauben   Spiked Helmets of Imperial Germany Volume 1   Spiked Helmets of Imperial Germany Volume 2.   Uniformen Der Alten Armee   Uniformenkunde Das Deutsche Heer   Uniforms and Equipment of the Imperial German Army -- 2 Volumes -- Woolley   Uniforms of the German Soldier   Uniforms of the Imperial German Army   Wappen und Flaggen des Deutsches Reiches und Bunderstätten   World War 1 German Army History The Advance from Mons   All the Kaiser's Men   Das Deutschen Heer in bunten und im grauen Rock   Fields of Memory   The First World War   First World War Germany and Austria-Hungary   The German Army at Passchendaele   The German Army on the Somme 1914 - 1916   The German Army on Vimy Ridge 1914 - 1917   The Guns of August   For King and Kaiser   The Pity of War   Storm of Steel   To Arms: Volume I Strachan   Through German Eyes   West Point Atlas for the Great War