How do I give the AI my own research or background material for a book?

Open the book, click "Codex" above the chapter list, and go to the Knowledge tab. You can paste notes directly ("Paste notes" opens a large text box) or add .md / .txt files from your computer ("Add .md / .txt files", up to 2 MB per file). Each entry has a "Send to AI" switch, so you can keep material in the Codex without sending it. Everything switched on is included every time the AI writes — chapters, regenerations, extensions and the outline — as authoritative facts it must not contradict.