My impression was that buying and storing books had become a compulsion. The article mentions that "he sold books primarily so that he could acquire more for himself". It also covers how he "deployed every little sliver of real estate" to 1.5 miles of shelving. To me this sounds like an obsession. It's possible that indulging the obsession made him happy but my understanding is that this is unusual.

It is perfectly possible I got the wrong impression here. I don't know him. All I can be sure about is that this was the impression I got reading the article.