Book Club Associates
Founded in 1966, Book Club Associates (BCA) work with other publishers and suppliers to offer a range of well known books at sometimes very low prices. Before the days of the internet they used to send a magazine out to people with a different selection each month - often discounted by quite a margin but with the caveat that if you didn't select something they'd sent you the "book of the month" choice.
Nowadays with the easy access of the internet this practice has died out and instead their website appears to be the main driver of business however they do still have some pretty good deals on there from time to time.
Mostly Harmless
The Hitchhikers Guide to the Galaxy Omnibus
Into The Labyrinth
The Hand of Chaos