As long as you have permission from the property owner and install your Street Library securely and out of the way of foot traffic, you are not likely to have any issues. Be sure to keep your Street Library in good condition (fresh paint, clean, accessible) to encourage people to use it and avoid complaints!
Some Street Librarian have decided to take out personal or commercial liability insurance as a safeguard, but most do not. If you are worried, consult a lawyer for a comprehensive answer.