The two English sentences have different meanings. "This book is best" means it's the best book of a limited set; for example, in reference to a series of books, you indicate the one that is best [of the series].
If you say it's "the best," you're literally saying it's the best book in existence; colloquially, you're just saying it's a good book. This distinction applies whether you're talking about books, people, or anything else.
The meaning of that is more or less the same but it's still not quite a perfect translation here. The main reason is the relationship between "this" and "book" in the sentence:
[this book] is (the) best vs.
[this] is (the) best [book]
--In the first one, [this book] forms a unit that stays together, just like in
[यह किताब] सबसे अच्छी है, as opposed to
[यह] सबसे अच्छी [किताब] है
Hope that helps!
When you say something is best, it automatically suggests that the thing better than every other thing of similar kind. So "of all" would make it look like the thing you just called "best" is better than any other thing in the world. But here it is only stated that it is better than any other book. Hope this answers your question. If this confuses you, i apologize. Please reframe your query and i will try again.