You could do a lot worse than starting with Karl Kiesewetter's large volumes on the history of German occultism. Since he died in 1895, all of his published works ought to be in the public domain by now, and available online.
For more recent work check out the books and articles of Hans Thomas Hakl, who is still alive, and has built an enormous library of primary documentation..
Wouter J. Hanegraaff is focused more broadly on esotericism than occultism. He had made a lot of his publications available on academia.edu, where one can downlaod them for free. He usually writes in English. He is one of the editors of Brill's excellent two-volume Dictionary of Gnosis and Western Esotericism (2005), which covers German esotericists along with other Western ones.
Re: History of German occultism
For more recent work check out the books and articles of Hans Thomas Hakl, who is still alive, and has built an enormous library of primary documentation..
Wouter J. Hanegraaff is focused more broadly on esotericism than occultism. He had made a lot of his publications available on academia.edu, where one can downlaod them for free. He usually writes in English. He is one of the editors of Brill's excellent two-volume Dictionary of Gnosis and Western Esotericism (2005), which covers German esotericists along with other Western ones.