Lugers and Walthers have always been great investments,if you want to "dip your toe" in those waters I would suggest two Books,the first, Jan Stills Third Reich Lugers;
http://www.amazon.com/Third-Reich-Lugers-Jan-Still/dp/189351305X 
Also another by Jan, Imperial Lugers;
http://www.amazon.com/Imperial-Lugers-Jan-C-Still/dp/1893513106 
Both are out of print and hard to find but well worth the price,another older one that is helpfull is Lugers at Random -Charles Kenyon,it does pop up on ebay and amazon at times.
Be careful,lots of boosted ones out there,much more fakery then the double gun arena.
