Timing: First 1/2 of June is close to peak and will likely give the best paddling options. May 15 - June 1 there are options but cooler weather can slow things down and you might be facing low water. After June 15, there is still water into July, but things are trending down. If you want the best action, show up first or second week of June.
Guide book: whitewater of the southern rockies is the most recent guidebook that has loads of info and has really good flow beta for min/med/high etc.
has online writeups for all of the colorado runs you could be interested in to get your appetite going. Good resource as well
Hooking up with locals should be no problem for most of the major drainages mentioned. Many boaters use the trip planner fuction on here to hook up with other boaters to share shuttles etc.