Ive done the C2C a couple of times in early September, which was ok weather wise but also had a days downpour too.I think may is also a good month, can be wet in june, however you never know with our weather do you.As for guidebooks, Ive got the Steadman, Martin Waiwright, A - Z, and Alfred Waiwright.Along Swaledale, you can opt for the high level route from Keld to Reeth, which is by far the most interesting, or the Low Level route as far as Gunnerside. From Gunnerside to Reeth, there is the continuation of the Low Level route that follows the river most of the way, or the Mid Level route ascending out of Gunnerside, which is the least interesting of all.The A - Z gives OS maps 1:25,000 which I prefer and is excellent. It's only fault imo is that it follows the Mid level route out of Gunnerside, as does the Steadman, the others follow the Low Level riverside route to Reeth.What I would do is follow the High level route from Keld, but if you want to do the Low Level one then I'd simply follow the marked route to Gunnerside then follow the ROW alongside the river to Reeth, then back on the marked route from there.To be sure of accomodation next year you need to get booking soon.