The crossword solution for the term BOOK, SUBSTITUTE is RESERVE. The term RESERVE can be used in a variety of contexts. In general, it means to set aside or keep something for future use. For example, a library may RESERVE a book for a patron, meaning that the book has been set aside for them and no one else can check it out. It can also refer to a person or thing that is kept in RESERVE for a special purpose, such as a RESERVE player on a sports team. RESERVE can also mean to hold back or restrain oneself from doing something, such as reserving judgment until all the facts are known.