For me, it's this one:
That's Once Upon A Galaxy by Alan Arnold, a diary of the making of Star Wars Episode V: The Empire Strikes Back. I got this in 1980. In fact, I think I even know where I got it: the tiny bookstore at one end of the Beaverton Mall in Beaverton, OR. At least, that's where I think I got it. I may be completely wrong on that score.
So, what's yours? Remember: this is what physical book has been in your collection the longest. Not which book is oldest!