For years (during D&D 3.0 and 3.5) I followed Monte Cook and most of his work. But when D&D 4th edition was announce I lost interest and lost track of role playing games . . . until a few weeks ago when I read "Reality is Broken: Why Games Make Us Better and How They Can Change the World" by Jane McGonigal. That book rekindled my interest in role-playing games.
Then, on Google+, I saw an announcement about Ptolus being available at DriveThru RPG as a PDF. I bought it because I wanted to see how how Mr. Cook handled creating and presenting a game campaign within the d20 rules.
Not only did I get the kind of education I wanted, I got excited about possibly dusting off my D&D 3.5 books and running a pair of games in Ptolus -- one running the pre-made games like "The Night of Dissolution" and "Dark Tidings" to name two. The other group I want to run using my own stories created with some of the hundreds of story hooks Mr. Cook has seeded in the Ptolus Campaign Book.
One feature that absolutely blew me away is, besides having a useful index in the back, the Ptolus book has item specific index running in the side margin so something that is mentioned in the text (such as the Delvers Guild), has a page listing on the outside margin of the page where you can go to get a full description of that item, spell, group, person, location or whatever. It is useful for your first time through the book and doubly useful when you begin to make your campaign. Very clever.
The outside margin is also used for DM tips, information about how Monte used a particular item in his campaign and additional information supplementing the main text.
This is a great product.