For a basic introduction, there's a book by Frank Barrett titled Mercedes Illustrated Buyer's Guide or something on that order. It's a nice intro to the full range from very early to 1990s. Although there are a few intense cognoscenti who may disagree with him occasionally, I find it a useful reference and pleasant daydreaming fodder as well.
Once you start narrowing down what you may want, forums such as this one can be extremely helpful, and Google is definitely your friend!
