What is the author's style in Berlin Noir by Philip Kerr?

Author Kerr's use of language provides both credibility and enrichment in all three of the works. A number of terms are obviously relevant to the time period as well as to German idiomatic expressions. Thus, "the Alex" is a slang word for the local police headquarters; a "spinner" is an informant; "puzzler" and "nutcracker" refer to a safe cracker; a "mulemouth" is a smart aleck. Kerr obviously did his homework, as he is not German nor has he lived in Germany.