The speaker has just published a book, and holding it, he feels all sixty-nine years of his life running through its pages. He doesn't try to explain this ("There are so many mysteries"). Instead he drifts into a vivid memory from age three: bleeding from the throat days after a tonsil operation, being rushed to the hospital, and screaming while a doctor burned the wound and his father held him.
The memory is frightening, yet the poem tells it in a calm, plain voice. That calm is the point. Even in the middle of that pain, the speaker says, he wasn't only the child. Some part of him was watching the scene. And that same watcher is present now, seeing him hold a pen and write these very words.
The mood is quiet, reflective, and a little wondering. The poem suggests that identity isn't the changing body or the stories of a life, but the steady awareness that has observed it all.