As I rewatched this episode, I started wondering why on Earth Stan even owned this book. To the best of our knowledge, Stan has no children of his own. Not only that, but the book looks to be in very good condition, with no signs that it could have been from that time anyway.
Then I remembered this:
Stan knew very well what he was saying and doing, illustrated by how he was willing to discuss the birds and bees talk in the first place. Maybe it’s because he had practice with a certain kid who now looks up to him.