The Philosophy of Friedrich Nietzsche | H.L. Mencken
Despite being the first book on Nietzsche ever published in English (in 1908), this is still one of the better intros, and it's almost certainly still the most entertaining. It should be of interest to both Nietzsche and Mencken aficionados. This edition contains a new introduction by Charles Bufe, a new Recommended Reading section, and dozens of new explanatory notes scattered throughout the text.