I had an email request asking for a good (and free) online introduction to the thought of Charles Hartshorne (1897-2000). I have two introductions to recommend, actually. Beyond online introductions however I would recommend three books, which Hartshorne himself said composed an "introductory trilogy" to his work. Those three books are Creative Synthesis & Philosophic Method, Wisdom as Moderation,
and Creative Experiencing.
I would also recommend picking up the LLP volume dedicated to Hartshorne as well as Hartshorne's The Zero Fallacy.
Those five books together would pretty much round out a basic introduction to the whole of his work.
For free online resources I would recommend THIS
introduction by Donald Wayne Viney, as I've found it to be excellent. I hope it proves to be informative for After Nature
is the online version of a book which has proven useful to me in the past. The essays are all by Hartshorne himself. It also comes with a comprehensive bibliography.