Let it be as elaborate as you can: You can use up to 5000 characters.
Put yourself in the shoes of a ghostwriter: Explain everything that you would tell a human writer.
You may also include a suggested table of contents (TOC).
If you have a target audience in mind, state it.
Indicate any particular angles or ideas that should be covered.
Don’t worry about spelling or grammar – just ensure your thoughts are well understood.
Keep in mind that while this description will serve as a guide for the AI, it might not follow it exactly. The more specific the description, the greater control over what comes out at last.