What makes a physical book worth owning?

This week I began reading Mr. Penumbra's 24-Hour Bookstore by Robin Sloan, a real treat for anyone who loves books.  It was late in the evening when I began, so I only had just gotten into story when I started getting sleepy.  So I put out the lights, rolled over, and was surprised and delighted to see this.  Why did the author do this?  According to an NPR interview he did: "When you're making a print book in 2012, I actually think the onus is on you, and on your publisher, to make something that's worth buying in its physical edition."

I love books, but have been trying to not be one of those people whose lives are overrun by them.  Mostly because I always have about 30 library books out, and their due dates put them ahead of any unread books I simply own.  I never catch up!  But I make an exception for a truly special edition of a favorite book.  One example is Lamb by Christopher Moore, a hilarious yet reverent "lost gospel", which has been published in a special edition with a faux leather cover with gold trimmed pages...I really couldn't resist.

What are some books that you couldn't resist owing in all their paper glory?  Why specifically did you acquire them?