Legal Question in Criminal Law in New York

Re: stealing?

No.

Stealing or more appropriately larceny is the willful and deliberate denial of a person of their property without the use of force.

In this instance, while it is is probably the proper thing to do, i.e. turn it into the lost and found. The effort you expend to return the book to its rightful owner is just as good. So long as you realize that the book is lost or mislaid, which means its not abandoned. Property that is not abandoned remains the property of the rightful owner. So be prepared to return it when the owner asks for it, otherwise it is stealing.