This book was published in 1896 following the dedication and transfer of the Washington Arch to New York City authorities. The book narrates the history of the Arch from its beginnings in the imagination of William Rhinelander Stewart, through its construction in both wood and marble, and covers events such as the laying of the cornerstone and the dedication ceremony. I found this book very helpful for several reasons: lists of subscribers and committee members are included, letters written by Stewart are reprinted, orations given at ceremonies are transcribed, and there are many images. The copy at NYU does not have images, but the copy at the N-YHS Manuscript Department is an original version which includes all images. It can be found here: http://dlib.nyu.edu/findingaids/html/nyhs/washarch.html
This resource is great because it was written by those who were there and when the events were still fresh in the news.
The History of the Washington Square Arch. New York, NY: Ford and Garnett Publishers, 1896.