All you need to play the game is in the Keeper book, I've managed to run a successful campaign using just that, we started with one of the two very good premade adventures found inside and then went into homebrew, the investigator's handbook gives more options for the players and provides setting material for the 1920's setting to help inspire both GMs and players during campaign and character creation. There's more occupations, as well as more information about each one, and a few optional skills for them. Aside from that you don't need the Investigator's book at all to play the game. You can play just fine using just the keeper book and screen (which with how complicated some of the rules are, particuarly combat) I strongly recommend).

All you need to play the game is in the Keeper's Guide. I've managed to run a successful campaign using just that. We started with one of the two very good premade adventures found inside and then went into homebrew.

The Investigator's Handbook gives more options for the players and provides setting material for the 1920's setting to help inspire both GMs and players during campaign and character creation. There's more occupations, as well as more information about each one, and a few optional skills for them. Aside from that you don't need the Investigator's book at all to play the game. You can play just fine using just the keeper book and screen, which with how complicated some of the rules are (particuarly combat) I strongly recommend.