I have now finished two of the four in the big Darwin book.
Origin of Species was very good. Most of it is still relevant, and it's a great explanation of natural selection for a lay reader (as long as they're prepared for a long argument). There's a lot of interesting discussion of things that are kind of taken for granted in modern surveys. It's also interesting to see where the gaps in knowledge left him unable to complete certain pieces of the puzzle.
Definitely worth a read, especially if you have been kind of meaning to get around to it. I have for about ten or fifteen years now, and I should have done so sooner.