This past summer I committed to expanding my hip hop knowledge. I spent four months reading about and listening to primarily hip hop. I figured, I have the rest of my life to continue listening to the same music genres as always. ...but now I have an amazing curiosity for hip hop. Well worth it.
But for people getting into it -- there is a ton of garage out there to avoid. A great place to start is with another book:
http://www.amazon.com/Ego-Trips-Book-Rap-Lists/dp/0312242980This book has chapters listing the best hip hop albums and singles from the first 20 years of the genre. Great place to just pick an album and listen.