Yeah, 'The Sicilian' expands on Michaels time in Sicily after killing Sollozzo and McCluskey. Michael, Vito and Clamenza feature in it now and again, but the book itself follows the life of a peasant turned bandit/freedom fighter called Turi Giulliano.
The book itself is top notch, absolute quality. I wouldn't really call it a sequel as it takes place during the middle of the original book, but its definately worth a read.
Buy it now!!!