What the author means, is that many people read a lot of books, but don't internalize and adapt their actions accordingly in a truly meaningful way (count me in). So his argument is, to better take one good book and truly learn from it and change yourself than read a hundred books and forget 90% of what you read, let alone to change your actions.

That said, I agree with you that having more sources of information reduces the risk of falling to the misconceptions of one or a few authors.