when you ask someone this and you are talking in the persence of more than one person, how do you clarify who you are speaking to? this phrase could be "is this book yours/hers/his", correct?

Usually "¿Este libro es suyo?" without any other consideration, refers to "Is this book yours?" (polite way; if you use the familiar way it would be: ¿Este libro es tuyo? both refering to de 2nd person of the singular).
If you are talking about someone else (third person) and you want to know if the book is hers, his, theirs, then you usually ask: "¿Este libro es de ella? (Is this book hers?) or "¿Este libro es de él? (Is this book his?) or you use the name of the subject: ¿Este libro es de George? (Is this George's book?)
Another posibility is that you don't mention the subject (the third person you are talking about) and ask: ¿Este libro es suyo? but you point out at him or her.
Greetings.

Indeed if you're using the formal form you say ¿este libro es suyo?, there's no way to tell if you're referring to you-him-her, you'll have to use your eyes and/or finger (a blind person won't be able to tell).


saludos
IDEL
Aprender español en Lingq ---> http://spanish-lingq.blogspot.com/