The excellent booktrade newsletter book2book (booktrade.info) has the story:
Thousands of people descended on a Bristol warehouse in search of free books after a major Amazon supplier left the premises with millions of titles inside.
Bookbarn, the UK's largest used book warehouse, moved out of the 56,000 square metre premises after their lease expired. The company had been at the warehouse for about three or four years and managers of the site invited people to help themselves so it could be cleared.
*sigh*