A casebook is a detailed written record — a book or a file that works like one — kept by a doctor, a social worker, or a police officer of the cases they have dealt with: who the person is, what happened, what was done, what comes next. Each person or problem is a “case,” so the book is where those cases live. It is a workplace word, a little formal; you are not meant to leave it on the bus. Though actually, law students also use casebook for a textbook stuffed with real court cases to study. Same spelling, different shelf: one is a record of your own work with people; the other is a school book of other people’s trials. The sense here is the first, the file of cases you have handled.