Has anyone read the Saxby Smart detective stories by Simon Cheshire? My son has started reading them after the author visited his school (and was very good, apparently). These books are simply brilliant, they've got special questions in them so you can solve the mysteries for yourself. Some are quite easy, some are really tricky.
My son (just left year 5) and I are really enjoying them, they're also extremely funny. I reckon they're great fun for adults too, I keep telling friends about them!! We're having to buy them online, though, we can never see them in Waterstones. 