I was in WH Smith yesterday in the Metro Centre, and they had a number of double packs of paperbacks which were originaly two for the price of one at £6.99, but are now marked down to half price so thats £3.49 for two books.
There were a few different authors available, two that I know and have read books from were Ben Elton and Mark Billingham.
The Ben Elton set were Blind Faith and Chart Throb for £3.49.
The Mark Billingham set were Lifeless and Death Message for £3.49.
There were a few others around too, cant see anything on their web site so looking like this might just be in store.