I saw a summary of The Sicilian, and it basically suggests that it's a side-story in the God Father novel. I haven't read The Sicilian yet, but I just want to know if it is indeed as side story.

The summary says that it happened when Michael Corleone was in excile, and his father Vito tasked hime to look for The Sicilian. Now, I was looking at the God Father novel for passages that at least suggest that Vito did asked Michael to look for this Sicilian.

Not at all. Michael has a very small role in the book. Since Puzo had placed Michael in Sicily around the time of Giuliano's death, I believe that he decided to use it as a springboard for the story. The book is about Giuliano, the Sicilian Robin Hood. And it's great.

I read somewhere, probably here, that Puzo just wanted to write a novel on "The Sicilian" but his publishers insisted in some sort of link to the Godfather because it would sell better. If I find more on this eg. source, i'll post it.

Yeah I read that too. The part of Michael takes up less than a third of the overall book.

I hope the fact that you now know that Giuliano dies doesn't spoil your enjoyment of the book!!