If you would like to explore the city away from the beaten path. The following travel guide will help you find some of the more unusual sights. Unfortunately this is our only recommendation for now, as there is not that much travel literature out there about Cologne (not in English anyway):
We love this travel guide that has come out recently: Only in Cologne: A Guide to Hidden Corners, Little-Known Places and Unusual Objects. Duncan Smith presents unusual sights, hidden corners and sometimes scurile objects. It includes Napoleon's cemetery, a road beneath the Rhine, the city lighthouse (which is not far from our DaHeim) or the place where the Red Baron learned to fly.