The title is “One Good Friend.” It was about a boy who makes friends with a wolf. That’s all I remember about it other than it was a hardcover book with a greenish colored book jacket. I read it many times in elementary school during the mid to late 70s.
Lost Classics of Teen Lit: Name That Book! 
ONE GOOD FRIEND
Sage, Michael
Published by NY: Cobble Hill Press, 1968
The story of a lonely boy and and a lonely wolf in the Central Park Zoo that become friends.
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