I don't know if it is even in a book or not, but in my siggy in one of my email accounts I use what I thought until today was a quote from him
"---- Say what you mean and do what you feel, because those that mind
don't matter, and those that matter don't mind. - Dr. Seuss -----"
I just looked in a Wiki and found out that is wrongfully attributed to him
https://en.wikiquote.org/wiki/Dr._Seuss
Misattributed
- Those who mind don't matter, and those who matter don't mind.
- Bernard Baruch in response to a question by Igor Cassini as to how he handled the seating arrangements at his dinner parties, as quoted in Shake Well Before Using: A New Collection of Impressions and Anecdotes Mostly Humorous (1948) by Bennett Cerf, p. 249; the full response was "I never bother about that. Those who matter don't mind, and those who mind don't matter." This anecdote is also quoted online at Chiasmus.com.
It has also become part of a larger expression, which has been commonly
attributed to Dr. Seuss, even in print, but without citation of a
specific work: "Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind."