In 1993 Cassidy wrote Saturn: A Beginners Guide to Using the Internet, followed by Stickman's Way Cool Guide to Network Wizardry. Cassidy published two additional Technology books, The Concise Guide to Enterprise Internetworking and Security and Introduction to Windows 2000 Network Administration. He co-wrote the paper "Can You Trust Your Email?" in 1993, warning of a flaw in the protocol used to deliver email, which could allow information to be forged.
Cassidy's "Photo-a-Week" project lets viewers into his life on a weekly basis starting on January 1, 2000.
His work with cutters and homeless orphans presaged his 2004 fascination with American gun owners which led to the book, Armed America: Portraits of Gun Owners in Their Homes, which provided a view into the lives of a controversial culture, praised by advocates of both gun control and gun ownership. It was named by Amazon as both one of the ten best art books of 2007, and as one of the 100 best books of 2007.
In October 2013, his poster and photo for the Curio Theatre Company's production of Romeo and Juliet led to an interview published in the New York Times.
In 2014, Cassidy's photoessay, "This Is What a Librarian Looks Like", based on photographs taken at an American Library Association event, was published on Slate.com. He continued to photograph librarians and libraries in the following years, culminating in the publication of a 2017 photobook, This Is What a Librarian Looks Like A Celebration of Libraries, Communities, and Access to Information.
