This works great with 5e and older versions of D&D (and OSR titles). You play one character with a sidekick through any published adventure. Really, I can't see why this can't be used with any ttrpg. We solo gamers are resourceful and very clever after all. Just for the heck of it, I might like to try Dm Yourself with the classic Marvel Superheroes rpg sometime!
This book is filled with good ideas for running your game. If you play solo already or want to try it, I highly recommend this. The book itself looks very nice too!