There's the factory manual "Seicento Technical Manual" free in the Downloads here -- download all 3 bits, pop into the same folder, use an unrar tool from Sourceforge (Google will find) to open.
Alternatively, Porter did a rather clunky FIAT manual which covers the Sei and lots of other stuff.
But I'd go with the Cinq Haynes (always some cheap on eBay) and the Seicento Technical Manual.