As already mentioned you will need german to understand the specifics because the language used in let's say federal court rulings do not go well with automatic translation systems. The results are often not only hardly readable, but also have changed meaning.
Apart from this problem there are other potential hurdles. One is called federalism, but first things first.
The Fundamental rights as given in the German Constitution (here#Katalog_der_Grundrechte_und_grundrechtsgleichen_Rechte_im_Grundgesetz) in german) were set after an era of injustice and police suppression. Thus they took privacy in high regard and as a rule of thumb intelligence gathering is forbidden if not especially allowed by law (so called "Eingriff" or "Grundrechtseingriff") and those laws are regularly challenged in court.
Some examples:
In regard to preventive measures each german state has their own "allgemeines Gefahrenabwehrgesetz", which is conveniently named totally different in each state. Plus there are federal laws regarding federal authorities e.g. the "Zollfahndungsdienstgesetz". All of them may allow more or less in one regard or an other. But all of them must be differentiated from the repressiv measures of criminal procedure found in the "Strafprozessordnung".
For intelligence gathering relevant are also specific federal laws like the "BND-Gesetz" or the "Bundesverfassungsschutzgesetz" and for good measure state level "Verfassungsschutzgesetze", because every state has their own domestic intelligence and security agency as well. All with different boundaries. For all of those you have to keep the "Trennungsgebot zwischen Polizei und Nachrichtendiensten" in mind, which makes a comparison with other countrys even harder.
Finally there were some specific laws like the "Gesetz zur Beschränkung des Brief‑, Post- und Fernmeldegeheimnisses" which target just one specific issue.
That should be enough for a start.