What is an API key and why does BookyAI need one?

An API key is a long password that lets BookyAI use an AI service in your name. BookyAI does the work of planning, organising, formatting and exporting your book, but the writing itself comes from an AI service such as Google Gemini. You create the key once on that service's website, paste it into BookyAI under Settings, and BookyAI keeps it encrypted on your computer. You never type it again, and it is never sent to BookyAI's servers. You need only one AI engine to write books, and the Gemini key is free. From version 1.9.17, every key in Settings has a button with step-by-step instructions.