PPM does a great job introducing solo concepts and tailoring a starting toolkit. Solo is very personal, and no two players solo the exact same way.
OSR Cosmic Horror Solo is one of the broadest products on offer from PPM and gives a really nice starter toolbox. You get a yes/no oracle, a twist mechanic, complex questions, a conversation (tone) oracle, a scenario generator, the oncoming horror mechanic and lots of advice for how to solo, how to solo and investigation, and how to solo the genre. Again, this is a ready made kit for playing OSR eldritch horror games - and it works just fine out of the box. I really appreciate the explanations of how to use the tools, and the optionality discussed.
Because ultimately, solo roleplaying for most people is assembling a variety of tools for different use cases. Here's a handful of tools that have broad applicability that are dialed in for the genre, and so using just and your favorite OSR rules, you've got a solo game. Or, you can take pieces and mix in other tools easily.
I love the oncoming horror mechanic - a suit of cards represents a force acting against you, but the Ace is the trigger. I also appreciate how straightforward and relatively simple the investigation mechanic is here. This is for emergent mysteries, not traditional pre-plotted gm-led story puzzles. PPM products are scene based, so the tools are built around having discrete scenes. This works for me, but I do know not everyone likes that format.
If you are at all interested in solo (or just GM less) play using some combination of OSR, Cosmic Horror, Investigations, then I can easily recommend this product as a starting point or a supplement to raid.