I love antiquarian books, and it's probably my worst vice (there are others as well, unfortunately!). These are the ones that probably are worth the most these days:
First London edition of Darwin's "The Descent of Man", published by John Murray in 1871, in 2 volumes, beautiful condition.
First American edition of "The Origin of Species" - unfortunately, the first British edition goes for about $80,000 these days, so it's a bit out of my price range! Even so, this one is a treasure that I'm lucky to have.
1674 edition of Francis Bacon's "Of the Advancement and Proficiencie of Learning: Or the Partitions of Sciences". This is printed on rag paper that has held up amazingly well, with almost no foxing. It's in a half calf binding that was re-done about 100 years ago, with the original boards. This copy was owned by Thomas Henry Huxley, and it has his signature to the front page.
An edition of "The Tempest" with illustrations by Arthur Rackham, signed by the artist.
Good thing books aren't fattening...