This was said by Austin Phelps, an American Congregational minister, and I think it is a wise saying.
While there’s no doubt a lot of knowledge, wisdom and inspiration to be gained from a book, I think some readers read to avoid thinking for themselves. They will take whatever idea is presented in the latest book they’ve read, sometimes changing their opinion on important topics just to match the ideas presented there.
While I’m all for keeping an open mind and allowing myself to be swayed in my opinion on things if I believe there is good reason, I think it is also necessary that we all hold ideals that we have concluded after reading many different things, and neither hold simply to one without ever swaying no matter what argument is presented, nor allow ourselves to be swayed by every word and opinion we hear.
It is a difficult line to walk, though, but I think some people would prefer others to do their thinking for them.