I'm not sure how many of the people I've played ARGs who might be reading this, but Stop a Murder - Complete Cases¹ is very much like an ARG in book form, so it can be played by individuals on their own time. It reads as a series of email back and forth between the author and an unknown person, who claims to be a murderer. There are five books in the series, How, Where, Why, Who, and When¹. Each book contains a dozen puzzles, ranging in difficulty, which will answer each of the above questions. There's a companion website where you enter your answers and which contains additional content.
My wife and I have started solving these, and have completed How and part of Where so far. We have the dead tree version, and the copy we have contains some errors which have impacted our ability to solve a couple of the puzzles. I don't know if these errors are limited to print issues in the printed Complete Cases version, or if they are editing errors that exist in the Kindle edition of the individual books as well. Luckily, the "email" include spoiler alerts to assist in solving the puzzles, and the companion website contains additional hints if needed, so we were able still able to get past these errors. For instance, one of the puzzles contains a formula, which ends with x(3). This reads as "x times 3," but it was SUPPOSED to read "x to the 3rd power," or "x cubed." The result is, of course, drastically different if you multiple by three instead of raise to the third power.
All that said, this has been a quite enjoyable experience thus far, and introduces my wife to some ARG-like themes, so it's worth checking out even if you're not an ARG player. And if you're a Kindle Unlimited¹ subscriber, they're available free as part of your membership, so what do you have to lose?
¹ affiliate links