I was feretting through "Foyles" in London, and a hurried Swiss tourist arrived who asked the sales attendant for a "book about London" which would allow him to make the most of his 3-day visit...

Certainly not a good idea - given that it would take about 3 days to faind any given book in Foyles! I've always thought of Foyles as a sort of adventure/playground for bibliomanes. You go in without expectation of finding anything in particular, or anything at all. You wander, braving the narrow escalators, the dingy staricases, the dusty book shelves, the eccentric arrangements, and then suddenly (like wossname upon a peak in Darien) you come across, say, Chapman's Homer. And it all seems worth while.