Whenever The New York Times invites me to do a book review, I look for an excuse. I’d rather spend my extra time writing books than reviewing them. But when the Book Review editors asked me to review Samantha Power’s “The Education of an Idealist: A Memoir,” I said yes, without hesitation. It wasn’t because I suddenly had time on my hands. And it certainly wasn’t because Power is a friend. I’ve met her only once — in her last week as Barack Obama’s ambassador to the United Nations. No, it was because I knew only one thing about Power — from afar. She was a table-pounding idealist and human rights advocate, and believed in using American power to protect innocent civilians and advance democracy. And lately I have struggled with that position.