Flappers 2 Rappers is a great book, chock full of information on the slange of each decade of the 20th century. I got interested in this book because I was doing some research on the 1920s, and found this guide to jazz age slang. The authors of this website have put together a great list of 1920s slang, but they do say that there is even more contained in one of their primary sources, Flappers 2 Rappers.
I believe a lot can be learned about the path of society through the study of changes in the language. I wish I could remember what the term was, but there is a scene in Little Women where Jo uses a word that is commonplace today, and her sister tells her not to use "slang" because it's crude. Obviously the word was just coming into usage at the time, and generally when a word is very new, it tends to be shunned as incorrect or "slang."
As another example, anyone who has read very early American literature (such as the writings of the Pilgrims who came over on the Mayflower) knows that at this time, spelling was not standardized. Writers would spell words as they pleased, sometimes even spelling words differently when used twice in a single paragraph. It wasn't until later that spelling became standardized, and there became "right" and "wrong" ways to spell things.
What about you? What changes in language fascinate you? What words do we think of as slang today that you think will make it into the dictionary someday as "proper" English?