And here's another useful link which describes German (and Austrian) signalling practice in the earlier epochs as well.

http://www.du.edu/~jcalvert/railway/germaust.htm

In terms of books, the two MIBA volumes are pretty comprehensive and Zintl's book is invaluable for discovering the details of Bavarian practice:

MIBA Report 17, Signale, Teil 1: Haupt- und Vorsignale
MIBA Report 18, Signale, Teil 2: Gleissperrsignale, Läutetafeln und sonstige Signale
Bayerische Signale und Stellwerke by Robert Zintl - out of print but obtainable second-hand

Paul, of (formerly Prussian-occupied) Lower Saxony