In a town like London there are always plenty of not quite certifiable lunatics walking the streets, and they tend to gravitate towards bookshops, because a bookshop is one of the few places where you can hang about for a long time without spending any money.
“Do not give dogs what is sacred; do not throw your pearls to pigs. If you do, they may trample them under their feet, and turn and tear you to pieces.Matthew 7:6 NIV