$1000/day is pretty standard, and might even be low for someone who has the expertise and communication skills required to write a book. Even if that person had not written one.
Agency rates bring an entire agency, though. Speaking from experience, companies will pay other companies multiple thousands a day for software implementations without a second thought but typically will fight paying a single person $250/hr to do the same job (even if it's a 1-person job).
Companies routinely pay agency rates this high.