I purchased the Book 80/20 Triathlon - where can I find the free Workout Library?

You will find the 80/20 Triathlon workout library here: https://www.8020endurance.com/8020-workout-library. 

Then  follow these instructions to add the workouts to your Garmin:  https://www.8020endurance.com/how-to-load-a-fit-file-on-your-garmin-device