Yep, 25 pdr cartridges, 8 to a box, they should have a divider riveted just below the top. Ammo boxes were 'service colour' in reality green or brown, usually the latter. Smoke rounds came in bright green boxes.
The letter indicates the original use. B for bombs, C for cartridges, M for mines, P for projectiles etc. Some boxes were used for many different things over time, B166 and M104 in particular.
No book afaik but a fairly good list on BOCN.
http://www.bocn.co.uk/vbforum/threads/27615-Consolidated-List-Of-Containers
Nice boxes btw!