Hello,
a good book is Kallenbach: "Der Gleichstrommagnet"
http://books.google.com/books?id=YaoClWVoO2EC&pg=PA46&lpg=PA46&dq=kallenbach+gleichstrommagnet&source=web&ots=DBPHditoYg&sig=tGPAl1Q3WVVj3emhtx-ujpHGemE
In it the network method for magnetic circuits is described on page 99f . This book...