These two books have been a superb source of research material about all things Sentinel. Neither are still in print but copies do appear from time to time on Ebay or the like - due to the rarity, don't expect them to be cheap! Try also Woodpecker Publications.
[1] The "Sentinel" - a History of Alley & MacLellan and the Sentinel Waggon Works: Volume 1: 1875-1930; W. J. Thomas, Joseph L. Hughes. David & Charles (1973). ISBN 071535924X, ISBN-13: 978-0715359242.
[2] The "Sentinel" - a history of Alley & MacLellan and the Sentinel Waggon Works: Volume 2: 1930-1980; Anthony R. Thomas, Joseph L. Thomas. Woodpecker Publications (Nov. 1987). ISBN 0863171400, ISBN-13 9780863171406.
The next two are still available from Woodpecker Publications:
[3] An album of "Sentinel" Works Photographs No. 1 Standards & Supers. Anthony R. & Joseph L. Thomas. Woodpecker Publications (1992) ISBN 086317 181 8.
[4] An album of "Sentinel" Works Photographs No. 2 DG, 'S', Diesel & Rail. Anthony R. Thomas. Woodpecker Publications (2010) ISBN 978 1905 304 363.
[5] The Locomotives of Croydon Gas & Electricity Works. J. B. Latham 1970.
[6] Instructions to Drivers for running and tending “Sentinel Cammell” Steam Rail Coaches and “Sentinel” Patent Locomotives. Sentinel Ref. No. 210 R/C 1927.
[7] "Sentinel" Patent Locomotives. The "Sentinel" Waggon Works Ltd. 1931.
