Still have my 3rd edition.
IMO, It's the scenarios, not the ruleset, that you want. The rules are based on the "simulationist" RuneQuest Basic Rules Set (?) system, designed to simulate realistic combat. However, this same system means that, if you fail a roll to find a crucial piece of evidence, then your scenario can hit a dead end (unless you designed it to be fail-proof). Nowadays, there are various more "narrative" scenarios, that focus on telling the story and setting the atmosphere.
Personally, I think you're better off buying the already available Chaosium PDF's through their own website or DriveThruRPG. If you play solo, I think there's an app licensed from Chaosium with CYOA. Chaosium's "Against the..." solo scenarios are *very* good.