The BSA Twin Restoration Manual by Roy Bacon covers BSA A7 & A10 Pre-Unit and BSA Unit A50/A65 Twins; Thunderbolt, Lightning, Firebird, Cyclone etc
We use this excellent book a lot at British Spares as a point of reference and for it's accurate photos.
The BSA Twin Restoration Book is seen as "The essential guide to the renovation, restoration and development history of all post-war BSA Twins. Plus a guide on how to recognise parts, improve specifications and maintain BSA Twin Motorcycles.
Paperback 240 pages of extensive data and 270 illustrations.