I read this book about 10 years ago. I think it was part of a series but I don't know how many books there were in total.
It was about a boy that lived in England and his house backed onto a river. There was a tree in the garden that I think was somehow magical - I can't remember whether he could time travel using it, or put notes into it that would be available to people from different times in history? The boy used the tree to interact with his ancestors.