I know these can be quiet expensive books,but there are some good choices in the 3 for 2 offer.
You see the discount in basket it's added straight away.
What I did was find books I wanted check the price else were
I found the Doctor Who Visual Dictionary @ £12.99 on the 3 for 2, so 3 of them( yes i need 3) works out at £25.98,that's £8.66 a book,which is the cheapest i've found it.
As it's £9.09 + P&P @ Amazon.
Also if you sign up for newsletter they usually send you a discount code.
This code 123vip gives you 25% off full priced books too.