If it's professionally-related (e.g. giving books to people who wrote for me) without question it is Eats Shoots & Leaves, as some years ago I gave copies to all my freelancers. https://amzn.to/2POdCis
@sjvn Yeah, if I was giving out books to my writer's today, that's the one I'd choose.
@estherschindler It's a great fun and helpful. But, these days my first pick would be Dreyer's English.