Irish illustrator and stained glass artist Harry Clarke (1889-1931) has fascinated me ever since I first happened upon his work in a beautiful edition of Edgar Allan Poe's "Tales of Mystery and Imagination". He's like an Aubrey Beardsley with ten times the craft, displaying almost inhuman skills for detail and composition.
You'll find lots of high-quality scans of his work HERE. Personally, I like his monochrome works the best. It's one thing to look at the image as a whole, but when you zoom in and start going over the details, you can get lost for hours and hours. Simply amazing.