Darn, missed it—I meant to mention that yesterday (Tuesday) was the 150th anniversary of the publication of On the Origin of Species, surely the most important book of the 19th century. Although this is one of those important classics, like Smith's The Wealth of Nations, that is seldom read even by its admirers, I have a pretty strongly-held opinion that the version to read, if you want to read it, is the first. Later editions contained considerable new matter by Darwin, but in essence he was, in those later editions, addressing points raised by the debate that followed the book's publication—joining the argument as it were, like a blog post that gets continually revised in response to the comments it raises...and in my opinion those revisions dilute the impact and impede the flow of thought and language that defines the character of the original book as a reading experience.
There's a Kindle version of the first edition available for 99¢.
Featured Comment by Ronny A. Nilsen: "The first edition is available free from the Gutenberg project. The copyright on this book has expired so it's in the public domain."
Featured Comment by BrianW: "Like a lot of people right now, I expect, I'm part-way though actually reading it for the first time. I'm finding it less tough going than I had anticipated. Darwin's writing is very lucid. I'm reading the Everyman hardback edition, which is the first edition along with The Voyage of the Beagle, plus an introduction by Dawkins. Maybe Mike can provide the Amazon links for that? It is really nicely printed and bound—the kind of book I didn't think you could buy new any more (I have no affiliation, etc....)."
U.K. link to the Everyman's Library edition (temporarily out of stock)