This is the 13584th most frequent German word.
deutsches
"Deutsches" means "German" (possessive or attributive form).
In this sentence, 'deutsches' is used as an adjective to describe the noun 'book'. It is inflected as 'deutsches' because 'book' ('Buch') is a neuter noun in the accusative case.
Ich lese ein deutsches Buch.
I am reading a German book.
Here, 'deutsches' is an adjective modifying 'auto' ('car'). 'Auto' is also a neuter noun, so 'deutsches' is the correct form in the nominative case.
Das ist ein deutsches Auto.
This is a German car.
In this sentence, 'deutsches' describes 'Essen' ('food'), which is neuter in gender. The word 'deutsches' is inflected for the accusative case here.