This is the most authoritative and up-to-date dictionary of English place-names available in paperback. From Abbas Combe to Zennor, it gives the meaning and origin of over 12,000 place-names, tracing their development from earliest times to the present day. This second edition adds 300 new entries, expands existing ones, and holds a glossary and extensive bibliography.
Pretty comprehensive. It also has a glossary of common place name formation elements, as well as a 25-page introduction to how English places are named, so if a place isn't listed you can have a go at figuring out what it means. Will keep it nearby when I travel.