Kudos to author Stephen Howe for jamming so much solid info on empire and its related topics (such as imperialism, colonialism, decolonization, etc.) into a book that clocks in at under 130 pages. I've read three of these Very Short Introduction series books; they've all been good, and this one is the best so far in terms of readability and quality of information. Some of the books in this series can be surprisingly dense despite their slimness, but I found this one to be a quick read. Howe is probably better at raising questions than he is at giving final answers, but I don't think that's a bad thing -- this is a messy topic. Think of it as kind of a primer, and you won't be disappointed.
Howe does an excellent job of defining some of the terminology related to empire, which is no small task (not least because the terms are often so misused, confused, and/or politicized.) Much of the book consists of comparing and contrasting the different empires found throughout history. Admittedly, he spends more space discussing the modern (late-19th/early-20th century) European empires than he does land-based and/or ancient empires. If the book has a single 'weakness,' that's probably it -- though in order to bolster the sections about land-based and ancient empires, Howe would have had to break the 'Very Short Intro' format.
As someone who studied the British Empire in grad school, I was always intrigued by the idea of comparative studies on empires on a grand historical and global scale. However, there is surprisingly little comparative work done on empires out there -- doubly surprising when you consider that, in the grand sweep of human history, empires have been much more common and have been around a lot longer (since about 4K years ago) than nation-states. Howe's Very Short contribution is a good starting point, and provides a lot of food for thought for anyone interested in this topic.
Oh, one last thing -- the Further Reading list at the end of the book is excellent.