a. This book is useful for you to understand the basic concepts of physics.
b. This book is useful to understand the basic concepts of physics.
Which of the above sentences are grammatically correct and meaningful?
It seems to me that (a) sounds fine and (b) doesn't. I think in (b) we really need the gerund (is useful for understanding). That is why I suspect that (a) is not really grammatical, but simply sounds correct.