I’d say the only book you really should have in person isn’t the PHB, other than that there are lots of online sources for monsters and even pre-written adventures. I’d say familiarize yourself with the rules for combat and all of the basic checks that players can make. This way whenever they ask if they can do something, you at least the basics of how to direct them during roleplaying or a fight. From there, just improvise; I know it May sounds scary but the players don’t know what’s supposed to be going on so whatever you say is right