What do you mean with book? For a custom trek like this, you won't be able to join a group, so are you looking for a private guide? You shouldn't have a problem finding a private guided trek in Cusco on arrival. If you mean you're looking just for muleteers and no guide, then it might be harder to organize. You'd probably have to find a muleteer in Cachora for the trek to Totora (or maybe even just Yanama and then car transport from Yanama to Totora). And then another muleteer/car for your continuations (up the pass and back, and then on to La Playa and eventually Hidroelectrica).
Im addition, I'm not sure how much a there-and-back to the Salkantay pass will add to your experience, though. I haven't done it, but there were hordes and hordes of people coming off that pass, and I don't think the scenery is all that different from the Totora pass which you come across on the Choquequirao-Santa Teresa trek. If you want more trekking, perhaps look at the Ausangate region instead?
If you want, I can pass you the contact info for the guy who guided us. He is an expert mountaineer and prefers tours that go high into the mountains, but knows all the trails around there like the back of his hand.