In my opinion, I would avoid the Haynes manual (they leave a lot out) and get the Bentley instead. This manual will go into great detail with good photos on how your car works. However, the Haynes is not a bad choice if you can't afford the Bentley price.
Amazon.com: Mini Cooper Service Manual: 2002,...
Also, you could take a class of some sort on auto mechanics if you are that interested in learning. Reading these forums is also a great help.
