[Mistwraith]

Spittlefields is possibly the least touristy market in london now, more used by londoners than sightseers, i guess it is more traditional, but stalls with books are becomming more common, you can get paperback pulp fiction 3 for £5 in my local market, they do tend towards light reading, but sometimes there are things that are not something the average reader uses for the commute .
It is ex-bookclub, overprints, bad prints, bad cutting etc, but they are still readable.

Library sell offs are still probably the best place though.