Here is a favorite reference of mine is this 1883 atlas which has been digitized by the University of Wisconsin:
|Title:||Atlas des Deutschen Reichs / bearb. von Ludwig Ravenstein.|
|Publisher:||Leipzig : Bibliographisches Institut, 1883.|
xxxv p.,  p. of maps : col. maps ; 42 cm.
Here is a recent post I wrote which briefly explains how to use the maps.