Terry Pratchett's The Science of the Discworld is not a regular Discworld novel. Sure the Wizards of Unseen University are there and there is a narrative (including Rincewind) but there is a fair bit more exposition on the subject of the origin of life from the "big bang" (that's not a porno BTW) to the rise of civilisation.

Also contains writings from Jack Cohen and Ian Stewart.

I'll post this out to anyone with an on site account who convinces me that they want it most.