Following on from London Bus Routes One By One: 1-100, London Bus Routes One By One: 101-200, London Bus Routes One By One: 201-300 and London Bus Routes One By One: 301-969, this fifth and final volume in the series takes a look at the route numbers with letter prefixes. They derive from the Reshaping Plan of 1966, whereby established trunk routes were broken up, and their outer sections turned into feeder services linking interchange hubs or Underground stations. As these new routes were intended to be operated by flat-fare buses, it was decided to distinguish them by using letter prefixes based on geographical area. Flat-fare operation proved unreliable and was soon dropped, though the lettered routes remained, and the system was applied to the normal network when it came time to further sectionalise bus routes because of London's insurmountable traffic.

As in previous volumes, a potted history of the routes and their routeing details are accompanied by up-to-date colour photographs showing the buses that operate on each route. Illustrated with over 190 colour photographs, this volume represents an up-to-date snapshot of the fascinating modern London bus scene as it stands in the latter half of 2021.