What resources on Linux for an old Windows user?

Leading Edge Boomer writes: A retired friend whose personal computing has always been with Windows was just gifted a laptop running some version of Linux. Probably he's not even aware that there are different distributions for different purposes. He seems open to learning about this different world. What recommendations might /. readers have to bring him up to speed as a competent Linux user? I really don't want to hold his hand, and he's smart enough to learn on his own.