"Have you already read the book?" This question implies that a book might have been read before the designated time. By asking the above, the person is confirming whether the preconceived notion is factual or not. For instance: Person A: We're supposed to read "Golden Gate" for this winter's book reading festival. Person B: Yeah, but that's like 3 months away. We've got plenty of time to do it. Person A: You're right, but, you know me and books. It's an awesome story: the main character goes on a journey to the California and meets up with his future love. Then, they - Person B: Wait! Have you already read the book? Person A: Well... yes. I couldn't wait after I read the synop. I know it's not due to be read until December, but, I read it in advance anyhow. "Have you read the book yet?" This question implies that a book might not have been read (or not have been started) and the designated time for the reading to take place has already past. By asking the above, the person is confirming whether the preconceived notion is factual or not. For example: Person A: We were supposed to read "Golden Gate" for the winter's book reading festival. Have you read the book yet? Person B: Yeah, it was an awesome story! I can't wait for the festival to start. Person A: I know. And the best part is that they're gonna have a live reading of the book to start the festivities. I'm definitely gonna need to go to it. Person B: Wait! Have you read the book yet? Person A: Nope. I'm waiting for the live reading. You know me! I'm not a big reading fan. There are other examples, of course, but this should explain the meanings and usage of the phrase. Good luck! I hope this helps.

By asking "have you already read the book?" is implying the person has read the book quickly (if their answer is yes). Asking "have you read the book yet?" is a general question.