There is a fantastic book by Bruce Fogle, D.V.M., a practicing veterinarian and a specialist in animal behavior. Although the book can be a bit dense, it is an excellent reference for anyone who wants to understand why his/her dog IS the way it is and why they behave the way they do. Here is a long(ish) link that will take you directly to the book on amazon.com. It has received over 6,000 reviews and has a 4.5 star-rating.
Here are some "factoids" you can learn about in his book:
1) "puppies can become bonded to almost any species prior to 12 weeks of age;"
2) it appears that there is world-wide consensus that pups ideally "are obtained about 7-8 weeks of age during the peak of their socialization period;" lest one worries that they have missed this time frame, he also asserts ...
3) "it is doubtful that there is any age after which a dog is not trainable;"
4) There are specific puppy 'aptitude' tests to assess 5 specific canine-related behaviors (p. 93-94)
5) "human attachments develop best between 6-8 weeks of age;"
6) "fear imprinting occurs between 8-10 weeks of age."
Perhaps this is why many breeders suggest that puppies be picked up by prospective owners by 8-9 weeks of age.
The notion of critical time periods was introduced by John Paul Scott & John Fuller in their work with guide dogs for the blind and was published in their work: "Genetics and the Social Behavior of the Dog"
Finally, I have been told by many animal trainers that another critical period occurs between 12-16 weeks of age, when puppies should be exposed to as many different experiences as possible (all positive) and as many different types of people, dogs, and objects as possible. The general rule is: 100 people and 100 dogs in 100 days.
Good luck with socializing your puppy or new addition to the family.