1. This book is very interesting.
  2. This book is most interesting.
  3. This book is more interesting.
  4. This book is much interesting.
  5. This book is too interesting.
    Are all these sentences correct?
    If any one is wrong, please explain the grammatical reason.

4 is incorrect. ‘Much’ requires something to quantify the amount:
The book is much more interesting/this book is much less interesting, etc.