I think he would know about the mistery of the building.
I think you are mentioning about "mystery". If you really are, I think it would be a mystery of something specific that relates to the building. For example, the mystery of a murder happened in this building.
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I think you are mentioning about "mystery". If you really are, I think it would be a mystery of something specific that relates to the building. For example, the mystery of a murder happened in this building.
Otherwise, I suggest that you could say "I think he would know about the stories of the building".