grammar - using "the" before "most" in a sentence

I found this book the most interesting.
According to a book, the should be removed. Why?
Shall I have to mention that of all the books that I've read so far, this one was the most interesting? (or even here "the" has been wrongly placed before "most")
what about the use of "the" before most in this sentence.
He is the most intelligent student of this class. (Correct I think)