Bismarck: A Life Jonathan Steinberg
Bismarck: The Iron Chancellor Volker Ullrich
Blood and Iran: Germany 1871-1918 (Ch1 1815-71) Katja Hoyer (Outstanding author)
Englanders and Huns James Hawes
Our German Cousins John Mander
From Bismarck to Hitler 1890-1933 Geoff Layton (Access to History text book)
All the above books deal with more than just Unification. I will also issue a second booklist to cover Bismarck and The Home Front, plus the path to 1914 next time.