Trying to remember title of kids fantasy book. It was similar in style to the lion, witch and the wardrobe. Some kids find a magic ring which when worn transported you to a strange green forest full of hundreds of pools that lead to different worlds. The thing I remember about the forest was that if you spent too long in it you fell into a kind of coma-like sleep so you had to jump quickly into one of the pools and hope you landed in a friendly world that would help you in the quest (dont remember what that was) There was an evil witch chasing them throughout these worlds, there was a white flying horse in it too. That's about it - hope someone can help, thanks

http://en.wikipedia.org/wiki/The_Magician%27s_Nephew