Timeo hominem unius libri, Latin for I fear the man of one book, a phrase attributed to St. Thomas Aquinas.
The phrase is a warning against narrow-mindedness as well as against jumping to conclusions based on little evidence or just one point of view.
So, what's wrong with reading just one book on a topic, especially if written by a respected expert? The problem is, even an expert offers a limited perspective. And even experts disagree. To learn about a topic, one needs to study it thoroughly and see what different authors have to say about it. And, of course, one needs to give it one's own thought, draw one's own conclusions.
Ironically, for several centuries St. Thomas Aquinas was considered the only authority in his field of expertise (theology). All other authors simply repeated what he had said/written. Thus reading any number of books offered only a single perspective, clearly a situation Thomas himself would not have been happy with.