Tom has the closest thing to the line in the book. After the peace conference, Vito tells Tom that Barzini never committed himself and doesn't get caught on the losing side, and Tom replies, "You mean he was behind Sollozzo and Tattaglia all the time?". Then Vito calls Tattaglia a pimp, and says it's enough to know Barzini had a hand in it.

The whole course of the war is murky in the book. Barzini's not committed, Barzini's supporting Tattaglia, all five families (seemingly including Barzini) are actively at war. Which is it, when? It's hard to make sense of the ending if all the families were actively fighting the Corleones, but it seems they were. I assume that the explanation is that Puzo was writing quickly.