I've been doing some very basic maths so I could be wrong but it seems to me that if a Superpoints member gets 400 points for spending pre tax £10 then it would take a member spending a pre tax £100 to get 4000 points, which is the redemption cost of a £4.00 book.
That seems a really low reward point, and that's for a member, it's twice as bad for a non member