This map is as detailed as it gets for Santorini maps, and is the best one out there. Compared to all others I've examined, it has far more detail and is much more accurate. I have used it for walking on footpaths (mule tracks) with success. It also has good contour lines and colors. As a bonus, it has super-detailed street maps of Fira and Oia, showing buildings. Shops and restaurants are well indexed. Translations are often (but not always) provided into English, but they forgot to provide an English text for the museums in Fira, so these are difficult to locate on the map.
It's hard to find, and not available on Santorini at all (April 2011). I found a secondhand copy on Amazon.