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 readers. 
Also, 
THIS 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.