Because it's an object of the sentence.
English places the subject of the sentence before the main verb, and the direct object after the main verb.
Russian uses different case forms for this. The subject of the sentence uses the nominative case form (э́та кни́га), while the direct object uses the accusative case form (э́ту кни́гу).
So if I get it, what does matter here is that the action is acting on the book, so even if the subject is Кто reads it, we have to use accusative on книга. If I was saying "this book is interesting !" as there is no verb applying to book, I would use книга. That's it ?
Right! In 'This book is interesting" (Э́та кни́га интере́сная), 'this book' is the subject.
Who does read this book. Who is reading this book. Ambas estructuras son sinónimas.
There is a book on the table and you ask, - Who reads this book? Why - Who is reading...?
[ Who does read this book? ] ‧ is a perfectly formed emphatic inquiry!
Party A: Have you read Principia?
Party B: No. Have you?
Party A: Who does read this book? [ Philosophiæ Naturalis Principia Mathematica ]
"John does run." ‧ [ Tense ‧ The emphatic form of the present tense gives emphasis to the verb form, e.g., "John does run." ] ‧ ‧ Why Grammar Matters: Conjugating Verbs ‧ lawecommons.luc.edu/cgi/viewcontent.cgi? ‧ article=1112&context=luclj ‧
"I do know" ‧ ‧ use to do as an emphatic verb to help clarify or add intensity to the main verb. ‧ certain tenses, sentence structures, and ideas require a helping verb (also called an auxiliary verb) ‧ www.englishgrammar101.com/module-3/verbs-types-tenses-and-moods/lesson-4/helping-verbs ‧