In reply to interdit: I went to the Kullu Valley in mid April, it was too hot really so I would suggest late March/early April. If you can't get hold of the book in the UK, you can get it direct from the Authur if you search for him online. The book isn't perfectly accurate and you can't get hold of maps to back it up.
Another suggestion would be Gulmarg, I've not been but it's in Cashmere and there is a lift there which makes me think there maybe more on offer and locals who would be able to point you in the right direction.