1A book containing the correct answers to a test, examination, quiz, etc., used as a guide for marking or checking answers.
2A book answering questions about, or giving information on, a particular topic.
3A book or booklet in which students write the answers to a test, examination, etc.
answer book/ˈɑːnsə bʊk/ /ˈansə bʊk/
Early 19th century (in an earlier sense). From answer + book.