"A Dictionary of British Place-Names", compiled by A.D. Mills, is a unique resource for understanding the origins of place-names across the British Isles. Whereas many other place-name books are restricted to England or particular regions, Mills' scope is broad, extending to the whole of the United Kingdom, the Republic of Ireland, the Isle of Man and also the Channel Islands.
The book is organised alphabetically rather than by county or region, making it easily navigable, and includes more than 17,000 entries. It is not comprehensive, occasionally neglecting a few of the very smallest settlements, but for most practical purposes it will be sufficient. (Sadly many places which are now subsumed within Greater London have been omitted; these are detailed in the companion volume, "A Dictionary of London Place-names", also by A.D. Mills.) At the back is a glossary of common elements in place-names - an interesting addition - and there is also an extensive bibliography, covering particular regions or specialisations within place-name studies, as well as broader surveys on the interpretation and significance of place-names. This makes the book ideal as a foundation for delving into the subject in more detail.
Typically there are about 25 entries to every page, which means that not a lot of space is afforded to each one. Unfortunately this means that in many cases only the earliest attestation of a location is given. For example, in the case of London only its Roman name, 'Londinium', is given; the changes in spelling and meaning throughout the Anglo-Saxon period and beyond are not charted. This means that it is sometimes difficult to find the name relating to a particular period. In these cases it might be necessary to resort to a more detailed guide such as "The Cambridge Dictionary of English Place-Names". However, where a number of origins have been postulated for a particular place, it should be noted that Mills is careful to record them all.
Everything considered, "A Dictionary of British Place-Names" is an excellent reference, despite a few flaws. As someone currently writing a historical novel set in medieval England, I have found it tremendously useful.