Yes, he honestly thought there might be booby traps in the library because Crowley was vastly underestimating how great of lengths a bookloving libarian might go to in order to protect her hoard. Granted, he didn't think the boobytraps would be deadly saws, or swinging axes... maybe just a floor panel to open up and expell a naughty demon from the stacks. Something very benign and scooby doo.

Really, did Crowley not know who he was talking to? Aziraphale did everything BUT booby trap the bookstore against customers.

"It's not that far," Aziraphale tutted, giving both Crowley A Look and Alec a more encouraging, if exasperated one. He held out a hand. "We don't have all night, Alec. Give me your hand--Crowley, you take his other arm. Won't let you fall now, alright?"