A tree identification manual with a key is certainly helpful during a tree's growing season but not as helpful during winter. I have created a source that can hone you winter tree identification skills. Developing your ability to observe and size up a tree will be invaluable during the dormant season even as the leaf key is tucked away in your warm library. Get to know a tree and its parts, or "markers", and how these parts look in every season - especially in winter.
More winter identification information:
Persistent Oak Leaves - Photo by Steve Nix