I'd recommend the following:
Feldbluse - The German Soldier's Field Tunic 1933-45 by Laurent Huart & Jean-Philippe Borg (ISBN:978-2-35250-010-0) : A book with many photos on every page covering all the types of field tunics from pretty much every angle.
Waffenrock by Timothy J Curley & Neil G Stewart (ISBN: 932970-00-2-0) : If collecting Waffenrocks is your preferred area then you must not be without this book. Hundreds, if not over a thousand, photos of all the various Waffenrocks of the various Heer branches. Probably the bible on the subject in my opinion.
The Collector's Guide To Cloth Third Reich Military Headgear by Gary Wilkins (ISBN: 0-7643-1428-9) : Covers all the military services, insignia, the types of headgear, manufacturers, construction and reproductions.
Deutsche Soldaten by Agustin Saiz (ISBN: 978-1-932033-96-0) : New book and again loaded with photos on every page covering everything that the German soldier carried from tunics to can openers.
German Army Uniforms And Insignia 1933-1945 by Brian L Davis (ISBN: 1-85409-158-1) : A book that is probably on every collectors bookshelf.
Uniforms And Insignia Of The Luftwaffe vols 1 (1933-40) & 2 (1940-45) by Brian L Davis (ISBNs: 1-85409-106-9 & 1-85409-498-X) : Again, a book that is probably on every collectors bookshelf.
Badges And Insignia Of The Third Reich 1933-1945 by Brian L Davis (ISBN: 1-85409-512-9) : Shows examples of the many insignia worn in both photos and colour illustrations.
SS Steel - Parade And Combat Helmets Of Germany's Third Reich Elite by Kelly Hicks (ISBN: 0-912138-96-3) : Covers the subject matter from every angle.
German Military Cuffbands 1784-Present by Gordon Williamson & Thomas McGuirl (ISBN: 0-912138-74-2) : The must have book for collecting German cuffbands. Covers all the services and campaign cufftitles.