The very best source of information for German stamps is Michel's English language Germany Specialized. Maybe getting into two volumes too, which is the most expensive option therefore. You also need the skills to identify the stamps, just reading isn't enough. You have to understand watermarks, perforations and maybe have a good colour chart. Scotts covers Germany but in a more general way. If your collection seems simple or small I would continue with Google searches before I bought anything.
I think Michel has been printed in colour since 2005, so you might find used copies on
Amazon or elsewhere...