The Spare
The title of the book is itself a metaphor. It is a commonly used term to describe the second son of a British monarch. The first son is the heir to the throne. The second son is metaphorically a backup in case some premature tragedy should befall the heir. Within that recognized meaning is another one, however. At last, as the heir is living, the spare is basically meaningless. At least, relative to the royal line remaining unbroken.
Big Brother
Harry is the spare which makes his brother William the heir. “In some ways he was my mirror, in some ways he was my opposite.” One may become the King of England in the not-too-distant future while one is a Prince. They are still brothers, however, just like any pair of siblings. This metaphor is actually a little confusing. A mirror image is a reflection of whoever is looking into it, sure, but it is also the opposite of the reflection. The point is understood, nevertheless.
Soldiering
Harry is not merely a Prince or even just a spare, however. He is also, literally, a killer. "They were chess pieces removed from the board, Bads took away before they could kill Goods." This could be the ravings of a serial killer or a cop. It is instead a metaphorical rationalization of being trained to "other-ize" suspected members of the Taliban.
Bitterness
Much bitterness is spewed by Harry toward the press. “Centuries ago royal men and women were considered divine; now they were insects. What fun, to pluck their wings.” The metaphorical language here reveals the depth of bitterness. The comparison of members of the royal family to insects would be bitter enough, but the suggestion that the press is a bunch of sick kids who gain enjoyment from pulling off the wings of bugs kicks things to another level.
Diana
The woman who was Princess Diana or Lady Di to the world was just plain Mummy to Harry. "She was light, pure and radiant light, and how can you really describe light?" That he would describe his mother in such metaphorical terms is not that surprising. The rest of the world knew her longer, in a way, than he had. Light is an apt choice precisely because of its lack of tangibility.
