There are currently two books on the history of Harlequins:

Harlequin Story by HBT Wakelam, Phoenix House Ltd, 1954
The history of the Harlequin Football Club, formerly Hampstead Football Club, from the 1870s to the 1950s with information on their famous players, world travels and major games and tours.

The Harlequins - 125 Years of Rugby Football by Philip Warner, The Breedon Books Pubs. Co. Ltd, 1991
This book is a fascinating account of Quins with forewords by Will Carling and Peter Winterbottom and an introduction by David Brooks, Club President.