The 2022 PDGA Rulebook & Competition Manual is something you should definitely carry if you're playing in a disc golf tournament. This book is the template by which all tournaments both big and small are run. One thing that makes this sport amazing is that a stance violation at a C-Tier in Brazil is the same as it is on the DGPT at Maple Hill.
It only takes about 30 minutes to read from cover to cover and you already likely know most of the rules. But familiarize yourself with this and make sure you're making all legal decisions when you're out there on the course.
There are rules for penalty throws, how to hole out, what is considered a putt, what constitutes successful balance, how much time to look for a lost disc, and all of the other rules you need for competition.
One thing is for certain, every round of disc golf is unique. At some point something will happen where you've got no idea what the rule is. This small paperback fits in your bag and should help guide you in the right way.
They take up little to no space and you'll need it at some point. This is the 2022 version, updated from the 2018 version. So make sure you get your updated copy today!