What is an 'impersonal joy' in this context?

Solution 1:

My interpretation of this sentence is that his joy had no element of ego; Holmes had the joy of an artist who was too deeply involved in the process to be thinking "this is such a great work I have produced" or "my mentor will love this" or "this will make me rich and famous" or even "this looks great!". There was no thought whatsoever of "I."

Instead he was simply reveling in the simple feeling of joy like a small child at play, or or like a bicyclist coasting down a large, steep hill, or like a runner who has hit his stride, feels good, and is "in the flow." He had the same joy of a true artist so immersed and engrossed in the joyful act of making art that the rest of the world might as well not exist.

This is my interpretation, and I'm sure others may have differing interpretations.