Why did Michel buy the casino's in the novel? he was set after his fathers death plus in the book he sells the Mall not give it to Clemenza thus making him even more rich.Keep in mind Puzo probley did'nt expect a sequal. Thank you for replies.

Being rich in just one component of power. Power is the ultimate aphrodesiac.

I know I'm bringing a pretty old thread back to life, but I wanna chime in.

Michael's whole mantra was becoming "legitimate." Casinos, while considered a vice by many, were indeed legitimate, and if operated properly, were cash cows that didn't bring down unecessary heat from the police and feds. Picking up the casinos also enabled the Corleones to have more a cash stream to keep up with those families that jumped with both feet into the narcotics market, something the Corleones didn't wish to participate in.

...plus, there was an important synergy:

The Corleones were pre-eminent in illegal gambling. Nevada represented the only legalized gambling in America in the mid-Fifties. By becoming powerful in legalized gambling, the Corleones could leverage and extend their power in illegal gambling. The wire rooms, loan sharking, odds-fixing and other activities that they used to fleece gamblers in NY would help them fleece high rollers in Nevada.

Though GFII was not envisioned in the novel, we learn from the Senate hearings that Michael "controls all gambling in America." And he was angling with Roth to control it in Havana, the other legalized gambling mecca in the Western Hemisphere. That's a result of that synergy.

Which leads to the next question in that synergy: when he tells Gilday in GF3 that he has sold the casinos and anything having to do with gambling, do you believe him?

Well, remember the commission meeting where he distributes the profits from the casinos.

I think he did, otherwise he wouldn't have gotten that Papal Knighthood.

Yes, but he was distributing the final profits because the casinos were sold, correct? The Dons were all saying that they wanted to keep doing business with Michael, especially the Immobiliare deal. He even said their relationship was ending. I tend to believe him when he tells Gilday they are done with gambling.

As mentioned Michael wanted more money, legitimacy and power.
Ultimately both Michael and his father wanted legitimacy for their family (criminal and personal). So it was a sort of progression.

Vito has to muck around in numbers, bookmaking, extortion and other slimy activities. He's outside of the upperworld. But the money and power he builds up doing this is transferred to Michael who uses it to "purchase" legal casinos and hotels.

Michael straddles the upperworld and underworld but builds even more wealth than his father did. Then the next generation that comes after Michael is completely removed from any sort of criminal activity but has enough resources to own international banks, huge shipping interests, sports franchises, and so on.

A bit of a parallel:

"I must study politics and war, that our sons may have liberty to study mathematics and philosophy. Our sons ought to study mathematics and philosophy, geography, natural history and naval architecture, navigation, commerce and agriculture in order to give their children a right to study painting, poetry, music, architecture, statuary, tapestry and porcelain."

--John Adams

Exactly!