The Complete Dog Book Book

Even if you don't own a purebred dog, this book is a must-read. With close to 200 breed standards with representative photographs of each AKC-recognized breed, it's a definitive guide to dogs. If you've got a mixed-breed mutt, you can page through and find breeds that might be in his or her background.
What makes this book truly great, though, is the clearly written instructions and tips for dog owners, breeders, and lovers. Whether you're considering buying a puppy or you've got a ten-year-old couch hound in your house, you'll find something helpful here. Learn about what's normal in a healthy dog (temperature should be between 101 and 102.5), when it's time to call the vet about vomiting or diarrhea (if it persists for more than 24 hours, is very frequent, or contains blood), and how to teach a dog of any age the five essential obedience commands of Heel, Sit, Stay, Down, and Come. You'll also find instructions on how to train a dog to stand rather than sit (great for vet exams or grooming), how to administer medications (with non visual references that I appreciated tremendously as a totally blind guide dog owner) and more.
I only have one caveat. Don't binge-read the book from front to back like I did. If you do, you'll start on the common illnesses section and before long will start thinking your dog has at least three potentially serious diseases--when they don't! This is a book meant to sit on your coffee table for easy reference.
Just make sure your new puppy can't jump up on said table and chew up your beautiful volume of wisdom! (So there should be a sixth essential obedience command: "No!")