Why does Les say there is nothing more we can do after Christ's work?
Christ finished the work of the cross and rose victorious over the grave and over death. Therefore salvation is not earned by human works or repeated efforts. Scripture teaches that one simply believes Christ has done it all; no additional deeds are needed. Any message of 'do this and do this, then hope you're saved' contradicts that teaching.
Les explains that the book does not teach a list of human works—do this, and do this, and do this—and then hope you are saved. Instead, it teaches to believe that Christ has done it all. When Christ finished the work of the cross and rose victorious over the grave and over death, there is nothing more we can do. In the context, Paul rehearses Israel's rejection and the turning to the Gentiles, and the Gentiles believed and glorified the word of the Lord. Salvation rests on believing that Christ completed the necessary work, not on adding further efforts.