19th Century German Private Postage Rates?

Can anyone point me to a source of information on the postage rates of 19th Century German Private Posts? I can determine some of them by just looking at different covers, but I would like to have a more definitive and detailed listing of rates. Right now I am mostly interested in the rates of the Berliner PacketFahrt company, which was the biggest of the various private posts.

For those that do not know, German law allowed private companies to deliver mail within cities only, but not between cities. This ended on April 1, 1900, when the Reichspost was given a monopoly on the sending of all postal matter. There were 100s of companies set up to deliver mail within cities, may of which quickly failed or were created just to produce stamps for sale to collectors. But some where thriving businesses and received substantial compensation for the loss of business in 1900.