Hi FangFang
I agree with Pamela. I prefer:
A) I 'll return the book to you as soon as I finish it.
I think it depends a bit on the nature/meaning of the verb used. “Finish” refers to a definite point in time. I think I’d be much more likely to go for the present perfect using, for example, the verb “read”:
I’ll return the book to you as soon as I’ve read it.
Amy