A family of Microsoft word processing software products for creating web, email, and print documents.
Officially, no you are not supposed to use Home and Student to write a book that you will be getting paid for.
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Unofficially, MS doesn't usually hunt down the "little guy". They don't really have any way of knowing what you are going to do with this document.
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If you want to be strictly legal, you can find a onetime payment version of standalone Word 2021 "commercial". It will cost about US$150 (same price as home and student", or you can buy Office 2021 Home and Business. It adds Outlook and is a "business" class license for about $260.
Actually! If you are not using "fancy" features, you may find Word Online (free!) adequate, barely.
Other than that, you could get one of the 365 business subscriptions. But they are more expensive. And it gets a little more tricky with the email account.