I had a loved one who was seriously ill and has died. His bookkeeper has written checks for herself and other missuse of his money. What can I do?

Report it to the police as embezzlment

Also in the US, there are some areas with special elder abuse laws that include theft from their money. But as with any criminal, you call the police and report it.

The persons estate can also sue in civil court for the return of the money

On the other hand, if the checking account was joint with right of survivorship, then no wrong has been done. You aren't clear on how checks were written and what false pretenses there might have been - or not.