Empire Denis Judd
Empire Jeremy Paxman
The Decline and Fall of The British Empire Piers Brendon
Colonialism Nigel Biggar
Empireland Sathnam Sanghera
Stolen History (children's version of above) Sathnam Sanghera
A Cultural History of the British Empire John MacKenzie
There are many books on Empire. The older ones are of historical interest only. There has been a plethora of publishing in this area in recent years. Use your own judgments (plus reading of book reviews) to determine the objectivity or otherwise of any book that grabs your attention.