Both these books make fascinating reading but it's worth pointing out that neither of them is very helpful in tracing the history of the various map series. They are more concerned with the general history and politics of the organization. There is another book, which I have not yet read, entitled
Map of the Nation; a Biography of the Ordnance Survey by Rachel Hewitt published n 2010.
A useful book for tracing the lineage of the various series is
Map Cover Art; a Pictorial History of Ordnance Survey Cover Illustrations by John Paddy Browne published by the Ordnance Survey in 1990. It claims to reproduce an example of every known cover of every series of those maps used by the general public, and includes a brief description and date of the series.
I've occasionally found
Ordnance Survey Maps; a Concise Guide for Historians by Richard Oliver published in 1993 by the Charles Close Society for the Study of Ordnance Survey Maps
http://www.charlesclosesociety.org/ useful especially in connection with evidence relating to rights of way. The Society also publishes a series of monographs on aspects of the OS and these are listed in the bibliography.
Hugh
