I haven't played much Gumshoe. The thrust of that game system is that the mystery is there to move the campaign forward. That skills provided everything you need, but spending resources on those skills provides MORE information, more detail and more clues.
The mix into Pathfinder is almost like a skill challenge. The Game Master decides what type of skills might be used, if the players have it, they make the deduction automatically. On the surface, this might seem like it's going to blow away any mystery but here's the thing. In my own experience, especially in running a game like Warhammer Fantasy Role Play, I've seen scenarios where the character has to make some type of check or the adventure just comes to a crashing halt. That doesn't happen here.
the interesting thing though, is that the authors didn't just say, "Here's Gumshoe's basics" and allow you to run wild with it. They also provide some ideas on how spells would interact with the system. For example, Commune and other high level, absolute answer spells are mystified a little. While this might anger some, I can see the utility if you're running a Gumshoe style game in the first place.
In addition, the authors have provided a few quick role playing tools they call drives. This allows the Game Master to hang some campaign ideas on those drives and move the characters forward.
I bought my copy at Paizo and got the PDF/Print combo. Currently there aren't a lot of bonus materials for the set up, but there are new character sheets. This two page character sheet is customized for the Gumshoe Pathfinder bit and flips out the skills for you. I would like to see them add the other tools including the Investigator Matrix and Adventure Worksheet.
The former gives you a quick list to write down the players, their characters, and go over their Gumshoe skills that might be useful. The latter a quick worksheet for the GM that shows where he's put certain clues, and how many of them.
While the book is short, it does go over the types of games that can be run and even includes a quick Gumshoe style adventure for the Pathfinder roleplaying game set at 1st-3rd level. While some of the encounters seem a little challenging, I haven't personally run it yet so I could be reading it off, or it could just be that it'd be better off NOT running it with just 1st level characters. The use of the author to show what he's doing with Gumshoe and what style this particular type of adventure takes is a good look at how the reader might make his own adventures in this venue.
Internal art is all handled by Chris Huth. The artwork is solid and well done. The format is standard two column style. The maps are easy to read and use. Some blown up maps as a bonus download in the 1" = 1 square would be great but not vital.
The strength of the Pathfinder system is the strength of the OGL. If you have this and the 3.5 version of Unearthed Arcana you can run some pretty Cthulhu based adventurers as the characters wind their way through an investigation even as they lose more sanity!