The Rise and Fall of The British Empire by Lawrence James might be a good option. He gives a fairly unbiased opinion which I enjoyed. My one gripe is that it was a little too short (and it's over 600 pages lol).

Whew, that's a pretty broad topic, but I'll try to narrow it down with some stuff off my comprehensive list (although I admiteddly focused more on domestic England/Britain, rather than Empire (of course you can argue they are the same))

"Prostitution, Race, and Politics"- Phillipa Levine. Levine uses the control of venereal diseases throughout the British empire as a lens to view the concepts of race, gender, and empire. Good analysis of the "metropole vs. colony" relationship. (I also haven't read her "Gender and Empire" or "The British Empire," but she's essentially one of (if not the), experts on the British empire in the 18th-19th centuries).

"The Ideological Origins of the British Empire"- David Armitage. Armitage takes an intellectual approach to talking about how the idea of the empire was conceived and how people talked about an "empire," before the height of the the British Empire. May be a little early for you (I think it was 16-18th centuries), but might be worth looking at.

"Britons: Forging a Nation"- Linda Colley. I know Colley has kind of a love/hate, but it's a pretty influential work. Basically, she talks about the idea of the "Other" as a Catholic-France, and how Britons began to define themselves by what they were not, while also beginning to identify as an empire in the 18th-19th centuries.

"Ornamentalism"- David Cannadine. Again, a bit of a polemic (and some would argue outdated) work, but still seminal. Cannadine essentailly plays up Said's orientalist argument to look out how the British defined and thought of their empire through class, while also taking into account how the indigenous people they encountered reinforced their own ideas and formulations of class hierarchy.

"Civilising Subjects"- Catherine Hall. Hugely seminal work, Hall revolutionized the way we look at empire. Hall uses Australia/Jamaica juxtaposed against Birmingham in the 19th century to look at how race created a sharp divide between the civilized English and uncivilized colonial subjects. ("White Over Black" by Winthrop Jordan talks more about this hardening racial vocabulary. Doesn't have anything to do with the British Empire directly, but may be worth looking into if you go the way of race).

I don't have anything specific, but PJ Marshall does a lot on the British Empire.

That's all I can remember off the top of my head, but I'll take a look at my lists when I get home from work. Let me know if you want anymore info, or maybe need more suggestions for a specific topic.

Edit: Formatting