Maps to the great city of London
I usually carry at least two maps of London.
One is Streetwise London, a laminated, brochure-sized map that folds up, fits in a pocket (OK, so it pokes out), has an index, and somehow still manages to cover most streets in the center of London. Incredibly handy.
The other is the famous London A to Z , a compendium of every single little street, alleyway, and paved path in all of London. It is the cartographic Bible for the city of London, the well-thumbed reference work in every cabbie's glovebox, and lives in my day pack just in case I need it to pinpoint minor streets or want to explore outlying areas.