Click here to order "Sophie's World"
This is actually a little hard to get through if you're not totally focussed and interested, but it is definitely worth a try or at least a flick through. Even if you don't read all the history bits, it's a wonderful book.
Click here to order "The Solitaire Mystery"
This is a lot easier to read. It's like a fairy tale for adults, and it's a little confusing because of the number of bakers and which baker is which, but it's worth the slight confusion. This is also very philosophical.
Click here to order "The Christmas Mystery"
This is more a kids book. It's not really philosophical like the rest, but it's good (am I stating the obvious here?) and everyone below the age of about 18 should read it.