Turn-up for the book/books

A turn-up is slang for an unexpected stroke of luck and dates from the late 19th century, and when used in conjunction with book or books, it refers to a bookmaker's book, and derives from horseracing where the book is the bookmaker’s book or tally of bets. A rank outsider winning a race would represent a windfall for a bookmaker and would be a nice turn-up for the book. In America, the expression is more likely to be in the plural i.e. a turn-up for the books.