I know firsthand the FBI will try to get your guard down so that they can interrogate you by claiming they need your help with an investigation. Most people will want to be helpful.
If the FBI ever asks you for help, immediately call a lawyer. It’s worth the retainer to find out who and what they’re really after and to avoid any entrapment.
If the FBI is being nice to you, it’s probably you they’re after. They probably never wanted to hire the author as an informant. They probably suspected he was a terrorist and wanted to interrogate him without a lawyer present. It sounds like their plan worked.
Be smart. Call a lawyer. Let them talk to the FBI for you.