A revised and updated version of
Abraham Kuyper: An Annotated Bibliography 1857-2010 by Tjitze Kuipers (2011)
You can buy a printed edition of this book on the site of the publisher.
A letter to F.L. Rutgers is included in facsimile and in transcription opposite page 173. In this short letter Kuyper thanks Rutgers for his articles in De Heraut and makes several suggestions for subsequent contributions to the paper. Finally, he relates that the launch of De Heraut has gone very successfully and that the number of subscribers has already surpassed (by more than 1,000) the highest circulation figures of the paper in its former days.