Jimmiej, remember the old saying "Knowledge is power" ? That expression is especially important in the hobby of Luger collecting especially since so many faked markings are showing up and many refinsh jobs are very difficult to detect. If you intend to seriously get into collecting get all the references you can afford and it's easy to spend over $1,000 just on references. It's really difficult to say which ones would be the best ones to buy as all of them have tid-bits of valuable info. Here is a link where you can usually find most of them at widely varying prices. In the title bar type in Luger then search, then Lugers and search, that should get you most all of them that can be found among the various sellers.
http://www.bookfinder.com/ Good luck to you and your budget, you are on the right track.