This is the 13670th most frequent Russian word.
немецкую
"German" (adjective, accusative singular feminine form).
In this sentence, 'немецкую' specifies the nationality or characteristic of the 'школу' being visited.
Мы посетили немецкую школу.
We visited a German school.
The word 'немецкую' is used as an adjective modifying 'грамматику' to describe the type of grammar being studied.
Я изучаю немецкую грамматику.
I am studying German grammar.
Here, 'немецкую' describes the book's origin or language, being a modifier for 'книгу'.