In the Product Flyer of the famous book What is Mathematics, it is said that the book by Courant and Robbins is the third of the "Golden Oldies" combing with the books by Rademacher/Toeplitz and Hurwitz( "Dritte Staffel der "Golden Oldies" zusammen mit den beiden modernen Klassikern von Rademacher/Toeplitz und Hurwitz").
It is clear that the book by Rademacher/Toeplitz is The Enjoyment of Math (English Translation). I wonder what is the book by Hurwitz.
It's somewhat likely they mean his Lectures on Number Theory, which was a classic once upon a time. Although he was a very prolific mathematician, Hurwitz wrote very few books --- in fact, as far as I can tell, this is the only book he ever wrote.