Thomas More's Utopia
I'll refer to it as Thomas More's Utopia because, despite the modern meaning of the word, it wouldn't be for everyone. For a start, they have slavery. However, given it was written in the 16th century there's still a lot in there to learn from, and if you replace the word 'prince' with 'prime minister' I would endorse a big chunk of his advice to anyone ruling a nation (Messrs Cameron and Osborne, please take note. Or perhaps someone could give Ed Miliband a copy).
If you're not familiar with the book, it's couched as a conversation taking place between More and a well-traveled man who has found through vast experience that the most contented and best-governed nation in the world is the (fictional) nation of Utopia. He then explains to More and his friend why it is that Utopia is so great, and how it differs from England or other European nations of the time. This includes its justice system, foreign policy and welfare system, parts of which are now in place, parts sound ludicrously old-fashioned, and most of it I'm sure would have been thought mad (or subversive, or both) in the 19th century, let alone the 16th.
I believe it may have been written in Latin, so possibly depends on the English edition you get hold of, but I found it surprisingly easy to read, of great historical interest, and it shone a light on the great constants of socialist thought (for Utopia is a recognisably socialist nation, for all its oddities of antiquity).
Utopia is a book I vaguely intended to read in my late teens, but put off by the idea that it might be hard to read because of its age, and potentially long and dry (it turned out to be neither), I never got round to it. Being of Yorkshire birth and ancestry, naturally the prospect of free e-books piqued my interest so when I did get my e-reader a couple of months ago, I went straight to the online repositories of out of copyright books. A combination of resurgent interest thanks to Jonathan Rose's book (The Intellectual Life of the British Working Classes), and the ease of carrying several long books at once, plus a novel for light relief, means that I've been reading all sorts of books I never quite got round to at nineteen, as well as ones that weren't on my radar back then. I've particularly been enjoying John Ruskin lately, and for that alone I salute the rise (but never the domination) of the e-reader.