Carrie Fisher had at least one thing in common with Princess Leia, the character that defined her career — a dry wit. “If my life wasn’t funny it would just be true, and that is unacceptable” was Fisher’s “main maxim,” Sheila Weller notes in a new biography, “Carrie Fisher: A Life on the Edge.” This engrossing, gracefully written, occasionally hagiographic book doesn’t just repeat the motto, it illustrates it, recounting numerous tales about how Fisher, who struggled with mental illness and addiction, managed to find the funny in it all — and share that with audiences, both as an actress and a writer.