If you remove the definite article from your sentence it still makes sense but the meaning is different.
I found this book most interesting.
This just means you found the book very interesting.
Saying something is "the most" is a superlative. It identifies the book as being the most interesting of all books considered. For example:
I read three books, and I found this one the most interesting.
In this sentence, you are comparing the book with the other two that you read. As it is unlikely that you have read every book in existence, or that you would attempt to compare every book in existence, you would use "the most interesting" in a context where you had already defined what books you are talking about.