The book that helped me the most was "Retriever training for the Duck Hunter" by Robert Milner. It is an old book now and surprisingly very expensive. He has a revised book where he changed some of his methods but I found it very easy to follow and read and it took you step by step to a well trained retriever.
At that age let the pup have fun so very light when you are training. Train in ways that do not seem to be training. Sit before he gets his food, or fetch when you throw a small dummy and here when he is coming to you.
His method allowed me with little gun dog experience to train a dog to a pretty high level and by the time the dog was old enough for serious training, it already was pretty well trained and the last phase just really insured that the dog obeyed commands instantly.
This book will not help you train for field trials but if you want a dog that gets the ducks and geese you shoot and listens to commands then give it a read.