Indicating record loss

I need to cite a marriage record where the original record no longer exists in the County Courthouse. It is cited in a book of marriages done several years ago, and apparently did exist at the time the book was published. However, since then, the marriage record, an early "loose" paper, is no longer to be found in the Courthouse records; I searched for it in person a few years ago. How do I indicate that I am citing the derivative record because the original is no longer available?