Your best bet for an authoritative, well-referenced and relatively recent overview would be the five volumes of the Oxford History of the British Empire. The series divides Britain's imperial history into roughly chronological chunks. The first volume deals with the origins of empire, the second with the 18th century, the third with the 19th century and the fourth with the 20th century, whilst the fifth volume covers the historiography of the British Empire. There is also a companion series which covers thematic aspects of imperial history (such as Ireland and the British Empire, Gender, Environment, missionaries and so on) in more detail than the five main series volumes could deal with. The references within these volumes will also provide you with an excellent bibliography for further reading into the various aspects of imperial history covered within the series.