This book, AFTER You Get Your Puppy covers the last three developmental deadlines that your puppy needs to meet before he is six months old. 

4th Developmental Deadline — Socializing Your Puppy to People 
Your Most Urgent Priority is to socialize your puppy to a wide variety of people, especially children, men, and strangers, before he is twelve weeks old. Well-socialized puppies grow up to be wonderful companions, whereas antisocial dogs are difficult, time-consuming, and potentially dangerous. As a rule of thumb, your puppy needs to meet at least one hundred people before he is three months old. Since your puppy is still too young to venture out to dog parks and sidewalks, you’ll need to start inviting people to your home right away.

5th Developmental Deadline — Teaching Bite Inhibition 
Your Most Important Priority is that your puppy learns to inhibit the force of his bites and develop a "soft mouth" before he is eighteen weeks old. Whenever a dog bites a person, or fights with another dog, the single most important prognostic factor is the degree of bite inhibition and hence, the likelihood and seriousness of injury. Accidents happen. Someone may tread on the dog’s paw, or a child may trip over the dog while he’s gnawing a bone. A dog may snap and lunge at a person when hurt or frightened, but if the dog has well-established bite inhibition, it is unlikely the dog’s teeth will puncture, or even touch the skin.

6th Developmental Deadline — Continuing Socialization in The World at Large 
The Most Enjoyable Priority of dog ownership is to introduce your well-socialized puppy to the world at large. Your dog will only remain sociable and confident if he continues to meet and greet at least three unfamiliar people and three unfamiliar dogs every day. Meeting the same people and dogs over and over is not sufficient. Your dog needs to practice meeting, greeting, and getting along with strangers, not simply getting along with old friends. Regular walks with your dog are as essential as they are enjoyable.