Well first one would have to define what is good for them and then they would have to determine if every book they read conforms to that definition of good.
For me personally I would say good means any book that has some sort of positive impact. Whether that be significant, such as I learn a new skill or valuable bit of knowledge, or minor, such as I had a bit of enjoyment and relaxation whilst reading some pulp fiction.
So, has every single book I've read conformed to that definition? No. I've started to read books that I thought were crap and wasn't getting any enjoyment from. So I stopped reading them. No big deal, bit of wasted time is all.
I've also read books that I thought were very valuable and provided with me worthwhile and accurate information only to find out later on that the information provided was inaccurate. In one particular instance, operating under this false information caused some somewhat serious negative repercussions.
So in the second example, reading was not a good thing for me.
As a general principle though, I would agree that reading is good.