People can lose their lives in libraries. They ought to be warned.
Bellow isn't warning us about literal danger but about the seductive escape that books offer—the way a curious mind can vanish into reading and lose decades to other people's stories instead of living its own. The remark cuts against the romantic notion of libraries as sanctuaries; they can become hiding places where the timid and thoughtful avoid the messier, riskier business of actual existence. A librarian friend once told me she watched the same handful of regulars come in every single day at opening, sit in the same chairs, and leave at closing—and she wondered if they were reading their way out of lives that frightened them. It's a tenderhearted observation, really: Bellow isn't mocking the reader, but acknowledging how dangerous it can be to mistake intellectual nourishment for the full-bodied engagement that living demands.