The History of the Retriangulation of Great Britain
The History of the Retriangulation of Great Britain 1935-1962 is the definitive official account of the 1936 retriangulation.
It was published in 1967 and comes in two volumes. Volume 1 is a book of around 437 pages, including a number of photographic plates. Volume 2 consists of a box containing a number of maps of varying sizes.
The book, especially volume 2, is difficult to track down, and would certainly auction for several hundred pounds.
The Ordnance Survey have published the book online here.
A searchable version, with better quality scans, is available to the owner of TrigpointingUK, but is awaiting permission from Ordnance Survey / HMSO, or the expiry of the Crown Copyright in 2017, before it can be made available in this wiki. Search for 'Allen' above for a fair-use example of what is eventually intended.
NB. The book linked to above is in fact The History of the Ordnance Survey edited by W A Seymour, which is a different book from the subject of this wiki page.