“The last tie was broken.” Losing earthly attachments in The Call of the Wild

There's that famous line at the end of The Call of the Wild:

It was the call, the many-noted call, sounding more luringly and compellingly than ever before. And as never before, he was ready to obey. John Thornton was dead. The last tie was broken. Man and the claims of man no longer bound him.

Looking for a book that explores this theme (or where it's at least an element of the plot): mainly that a character loses their last earthly attachment and so is released, or freed to do something, whether good or bad.