"But horizontal wealth means not letting your increased income dictate your tastes. You like books and now you have money? Buy more books! Change those catenary bookshelves for good hardwood ones! In my case, build a library extension to your office. And, of course, you buy what will be useful for that most wonderful of pursuits, blind research, which is research without direction for the sheer joy of it."
(Original source is his book A Slip of the Keyboard - this linked blog post quotes that bit.)