Prince Harry is going to publish a book called "SPARE" in January 2023 (notice the "Half Price" discount lie as usual - it has never been sold at the full price but they're pretending it's a 50% discount - I'm sure this is illegal in many countries!).
But what does "spare" mean and how can you use it?
The book title refers to the fact that Prince Harry is the "spare" sibling (the extra brother/sister just in case something happens to the firstborn i.e. William).
“The heir and the spare” or “the spare to the heir” is a phrase that dates back to the days when death among children was common, and multiple offspring, or “spares” were needed to ensure a family's line in case the firstborn dies.
[heir = a person legally entitled to the property or rank of another on that person's death. Pronounced like 'air' /eə/ or /ɛː/ and rhymes with 'spare'].
= additional to what you need for ordinary use, or things that you keep ready just in case another one is needed.
e.g.
Similar words: back-up, reserve, second, extra, surplus
There's also an informal meaning, which is perhaps what Harry was getting at in the title of his book: