Lloyd Chambers is a pretty sharp guy who seems to know the area well. He occasionally offers scheduled small-group workshops, but I believe he also can be booked for private tours. Drop him an email.
I would think for the valley, anytime but summer would be good. It's too crowded in summer, and hotter as well. In spring you have the roaring waterfalls, in autumn you'll get fall colors if your timing is right, and of course there's the chance of snow in winter if you're lucky.
For summer (specifically July-Sept), my recommendation would be the high country, including portions of the Easter Sierra. This will get you away from the worst crowds, and there is some absolutely amazing alpine scenery to be found. Higher elevations also mean cooler temps in summer. While the valley is a sight to behold and should be visited at least once, it's the high country that really made an impression on me, and that's where I'll be returning on future trips.