I have been studying utilitarianism in particular in quite some depth as part of my university degree (PPE). And yes I would DEFINITELY recommend that book, it is excellent. Also, of couse, Mill's book itself is very important to read as it had such a significant effect on ethics and politics, though I wouldn't say that he is necessarily the best representative of what utilitarians generally believe (the devil's in the details).
Another very simple, straightforward & lucid book is Roger Crisp's 'Routledge Philosophy Guidebook to Mill on Utilitarianism', definitely recommended. Or anything by Crisp, on that matter.
If you're still hungry for more, still on level one/two is 'Mill's Utilitarianism: Critical Essays' by Lyons.
Sorry Luke I rushed that a little bit and didn't check before hitting 'comment'.
In economics I would say you should read macroeconomics and microeconomics separately, and most college-level textbooks are on either one rather than both anyway.
So Mankiw is definitely the best on Macro, whilst Varian is the best for Micro, but his is quite dry and mathsy, whereas for a Micro alternative Katz and Rosen is more readable but less mathematical.
So for Macro, go for Mankiw, and for Micro go Katz and Rosen if you can't handle Varian.
Hope that clears that up!
For elementary economics:
"Macroeconomics" by Mankiw, is without a doubt the best on the market. It is incredibly well written, and it's so good once you've read the book it fools you into thinking you understand absolutely everything on the topic!
"Intermediate Microeconomics" by Varian, is arguably the one to get. It can be a tad dry, and he uses lots of maths. If you don't like the idea of that then "Microeconomics" by Katz and Rosen is a very readable and less mathematical, though not quite as comprehensive as Varian.