As much as I find using PDFs of rulebooks obnoxious because every PDF reader I've ever tried chugs on all of the images in most of them, I don't have any in particular that I'd rather have taking up space on a book shelf. Lots of random, one-offs that I've accumulated over the years of my group wanting to test them out.
On that note, I've largely finished playing a game of GUMSHOE One-2-One, and... it didn't work as well as I hoped. At this point I can't decide if it's because of flaws in the game system, or just because I used it poorly. Very likely the latter.
My vague impression, after a single game with it anyway, is that it's very sensitive to how you set up encounters. That in turn is also very sensitive to how careful you are at steering the player toward the clues and conclusions you want them to draw, because you're supposed to set up encounters / challenges at specific moments and have detailed consequences prepared for them. As an example, if you want the player to resist flirting by an NPC, you're supposed to have a challenge designed with 3 outcomes, depending on how well they roll their Cool ability: an Advance for a high roll, a Hold for a middling one and a Setback for a low roll. Those are usually each accompanied by a Problem or Edge that you hand out to the player, so improvising is difficult. Even in a play by post format, I had a tough time thinking up appropriate Problems and Edges at the right time, since I wanted them to be something more interesting than a flat -1 to rolls or something. And I had to improvise a lot, since the player inevitably went off in an entirely different direction than I expected at the very beginning of the game and invalidating a fair bit of my prepared encounters.
On that note, I absolutely understand now why the game provides prebuilt characters for you to use in the scenarios it includes in the Cthulhu Confidential rulebook. First, you need to have a good motivation for a character to go and investigate whatever problem you set up as a GM, and having something to work with there helps. More importantly, a character who does not have an investigative or general ability usually can't even attempt it, so if you mess up encounter design and ask for one of those abilities to be used or rolled, you screw the player over. Did that twice by mistake.
Coming up with the right numbers for the challenges is hard too. As the GM, you know the player's skill levels so you can usually assign appropriate difficulties with a bit more confidence than, say, a fight in a D20 system game, but even so it's easy for a player to roll a 1 on a die on several encounters and end up saddled with lots of Problems that cause something of a death spiral. That's countered by making most Problems possible to get rid of by just "taking time", but if you don't impose consequences for taking time then it cheapens the effects a lot.
Anyway, the system works fine for what it does, but writing investigations for it is hard. A lot harder than the base GUMSHOE system intended for multiple players, I think. The prebuilt investigations in the Cthulhu Confidential rulebook are well designed and flavorful, and I think that's probably where the game does best: with a refined investigation that's been through rounds of play testing. It would take a GM that's very experienced with the system to create new investigations quickly and with any decent polish.