The correct answer is: no article.
The word "most" indicates that the book is being compared to a group of other books. In this case, the speaker is saying that this book is one of the most interesting books they have ever read. This means that there are other books that the speaker has read that are also interesting, but this book is one of the most interesting.
If we use an article, such as "a" or "an," it would imply that the speaker is only comparing this book to one other book. For example, if we said "This is one of the most interesting books I have ever read," it would sound like the speaker is only comparing this book to one other book that they have read. However, the speaker is actually comparing this book to a group of other books.
Therefore, the correct answer is "no article."
