I found this gamebook because not only is it one of the few modern-day gamebooks that has its own page on TVTropes, but said page is linked directly from the TVTropes page on the entire genre. Clearly someone's a fan.
You can download this as a PDF for free, but I shelled out the $4 for a paper copy because the angel on one shoulder whispered: support the author and the genre and the devil on the other whispered: it's really annoying to flip through the pages in a software reader. It's one of those gamebooks, like the Fighting Fantasy or the Lone Wolf series, where each passage is only about a paragraph long, so you're constantly changing pages. Even reading on an iPad, where you can flip through pages rapidly, felt awkward.
S. C. Cunningham, the author of Colefax Manor, does something a little bit odd with their book. Perhaps 80% of it, where you-the-protagonist explores a manor in the English countryside in the wake of a murder, is nearly static. There are characters -- suspects, I suppose -- who can be questioned but never move from their initial location or take any meaningful action at all. The butler is always polishing a bust in the library; the maid is always seated on a bed in the service hall; the gamekeeper is always napping in the stables. There are boxes you tick in your detective's notebook that keep track of various clues you've found, similar to the approach the Sherlock Holmes gamebooks take, but they serve almost exclusively to unlock dialogue.
It's impossible, so far as I know, to make anyone confess. Instead, like the Sherlock Holmes board game, I think you're meant to put together the means/motive/opportunity in your mind and then call back to headquarters with your solution, and you find out whether your case is strongly-supported.
It feels a little like a thinly-implemented parser game, which in my opinion plays against the strengths of the gamebook genre. A gamebook will never be able to match the simulation possible in a parser game, so why not lean hard into plot and character?
There is also a finale -- perhaps one could regard it as the Best Ending -- where you can take a more active role in resolving the murder mystery. It drops the investigative mood for more of a straight adventure feel, but it still feels parserlike -- the main decisions to make are avoiding instant-kill situations (which are adequately clued) and how long of a fuse to use on a bomb (the answer to which you can guess right now).
Despite the shortcomings I found in it, I would recommend Colefax Manor to someone designing choice-based interactive fiction, particularly if they're working in the gamebook genre. To me, it represents a particular school of thought of design; one I don't agree with, but one that helped bring my own beliefs into sharper relief.