Sorry, Belly T, I'm tired today. Here's the answer you are looking for:
Specific: It was John Lennon who wrote the book. General: It was John Lennon that wrote the book.
Here's the difference the way I see it. (I do not know what Swan thinks about the variation.) If John Lennon is the topic of the conversation, then speakers might use who as a specific reference marker. If John Lennon isn't the topic of the conversation, say, the conversation is about the book or even about some other famous person, then speakers might use that as a general reference marker to show that John Lennon isn't the topic of the conversation.
Note that, not every speaker feels its grammatical to use relative that with a person's name.