If you want to read a detailed history of the British Army, then Soldiers: Army Lives and Loyalties from Redcoats to Dusty Warriors should be your first port of call. The book is well written and makes a great read for anyone interested in military history. I really enjoyed it. However, it is exhaustive – or perhaps even exhausting; you can only read about so many regimental mergers without your attention starting to drift. I suspect that if you are interested in this sort of thing, you already own a copy and if military history is not your thing, you should probably avoid it anyway.