Which letter is typically used before a Dewey Decimal number to indicate a reference book?

The letter used before a Dewey Decimal number to indicate a reference book is "R." This designation is important in library classification as it helps patrons and librarians identify materials that are meant for consultation rather than for checkout. Reference books, such as encyclopedias and dictionaries, are typically intended for use within the library and not meant to be borrowed, hence the specific labeling with "R." This system allows for a clearer organization of library materials and facilitates easy access for users seeking reference information.