This is the 5182nd most frequent Russian word.
немецком
"немецком" means "German" (referring to the German language in the prepositional case).
Here 'немецком' is in the prepositional case, referring to the German language as part of the phrase 'на немецком языке' (in the German language).
Я учусь читать на немецком языке.
I am learning to read in the German language.
In this context, 'немецком' is again in the prepositional case, indicating the context of the language used.
Он говорит с акцентом на немецком.
He speaks with an accent in German.
The word 'немецком' refers to the language in which the book is written, again in the prepositional case.