I've pondered this question before, and I agree with the general trend of saving my laptop, irreplaceable campaign notes and hand-drawn maps over any purchased RPG books.
But if forced to chose a single printed book from my collection, I'd probably just let the whole lot burn!
That might sound strange, but I have a reasonably significant RPG collection -- about 3500 items at the moment. If I rescued just one book from the fire, I'd most likely eventually start collecting again. But, if my entire collection was wiped out, I simply wouldn't bother replacing any of it. I'd move to a purely electronic collection, and effectively start over with a clean slate. And I'd be fine with that.
However, if we include minis, then I'd grab two specific miniatures from the shelf. One of the members of our gaming group recently passed away under difficult circumstances, and the minis associated with his characters have quite a lot of sentimental value for our whole group. Sure, I could probably buy duplicates of those particular minis again online, but they wouldn't be the minis he used, so they'd lack the same meaning.