So now, leave the library alone and go look for something that isn't trapped. Or if you're feeling lucky, exit the library and send in a skeleton or zombie or unseen servant to start pulling books off the shelves and piling them outside the library in hopes of breaking any glyphs of warding/Symbols/etc. cast on the books. If it weren't trapped that would be a waste of precious time.
Obviously, if your DM's world never includes genuinely deadly traps, Find Traps is a waste of time--just as Meteor Swarm is overkill in a campaign where the DM never uses hordes. My worlds typically include both the standard "fun" kind of traps (made by trap gremlins), and the genuinely deadly "security" traps (made by intelligent wizards who are trying to protect their stuff and kill those who would take it). If, while in sandbox mode, you decide you're going to sneak into Archer's tower while you hope he's away and steal some medium-value stuff like old magic items, you probably want all the advantages you can get, and that includes Find Traps.