Scholastic mystery title from the late 60s/early 70s

I am trying to recall the name of a Scholastic book from the late 60s early 70s. The plot concerns a boy and his family who inherit a mansion and a mystery. There is a ghost in the story who helps his descendent locate a treasure hidden in the trunk of a tree over which a branch has grown. Lastly, there is also an elderly woman who was friends with the ghost when she was a child. I think the ghost's name is Nathaniel as is his descendent. Everyone remember the title?

I feel terrible - we just packed up my mystery/ghost story collection for an upcoming move! As I've been seeing them, I've been picking up YA/Kids supernatural-themed novel (like Jane-Emily), and your description sounds very familiar.
Not much comfort, but I understand your dilemma -I've been trying to remember the title of a anthology collection from my childhood with no luck!

There is actually a group on LT that helps people who are trying to remember the name of a book. You might try posting your question there:

http://www.librarything.com/groups/namethatbook

In order to have a better chance for results, be sure to follow the tips for posting on the group page.

Good luck!