Alphonse Mucha - pretty much amazing. Incredible draftsmanship, incredible color, incredible compositions. Pretty much one of my favorite people ever and a constant reference source for when I need something that looks amazingly feminine and graceful. I really can't think of any image of his I've seen that I didn't love or enjoy.
Of all of my books (art related) this spine is the most worn. I constantly love flipping through pages of his sketches and being enamored by their beauty.
His style is distinctive yet versatile and always lovely.