Continuity mistake: Right before Ben jumps off the cliff, he drinks a small cup of coffee and places it back on a saucer that Jonathan is holding. When the shot cuts, the saucer is still visible in Jonathan's hand, but the cup cannot be seen on it. The cup then reappears in the next shot.
Answer: Possibly because it was the closest building with height to it as they are about to be hit by a gigantic wave of water. There was no snow yet, so I don't believe burning books or snow was on anybody's mind yet. It turned out to be a great idea as snow soon starts to fall and those books were literally a life saver.
Susan D. Santos