Enrichment
I used the activities in these books to keep the dogs happy on a rainy day.
Canine Enrichment for the Real World by Allie Bender & Emily Strong
This book is dog-eared and underlined and stays in the dog snack cabinet in the kitchen. The authors provide so many practical and cheap ways to keep your dogs entertained, and the activities are also divided into beginner, intermediate, etc.
Get the Ball Rolling
This book is a great introductory guide to Treiball. Although Gussie and I ultimately gave up on Treibbal training, this was a helpful step-by-step guide with picture.
101 Dog Tricks
Porter worked his way through a lot of these tricks, including "fetch beer." This book is a solid introductory guide to all of the different activities you could potentially do with your dog.