If your a treasure hunter, then you love maps. The older the map, the better. I have maps from the early to mid 1800's covering the walls of my office. These maps serve a couple purposes, one they are just great decorations and two they are good research tools to the areas I metal detect.
Sometimes it is a struggle to have access to maps for a particular area, I often make my own out of frustration. The USGS Historical Topo maps are great maps from the 1800's and up. The problem is a lot of maps are missing (at least here in Virginia). I ran across the USGS Historical Topo Map Explorer website few months ago. The site has a tool that allows you to overlay current maps with their library of historical maps. Check it out here.
Good luck and HH