I didn’t realize that he was still alive. I read just about all his work as it came out, and have the first editions on my bookshelves.
I especially liked his collaborations with Jack Williamson and Cyril Kornbluth.
“The Space Merchants” was one of the first science fiction books I read.
He was one cool guy. "IF" you wrote him a well crafted and neatly typed (or written) letter he would answer in kind. Especially if the subject of your letter was a query about some of the science he wrote about. When I was a teenager I wrote him a letter that my science teacher prompted me to write and got an answer. (My teacher made me re-write it several times until he was sure it was free of mistakes)
Sadly I left it at my Mom's house and it got ruined when her basement flooded.