Look in the section "New Collector' and look towards the top lines, they are called 'stickies' and have much information on them, to include many book recommendations. Be advised that many folks, including myself refrain from saying, stay away from so and so book, as it is poor....
http://forum.lugerforum.com/forumdisplay.php?f=133
You need to know the general idea of what you like in a luger, or just buy some of the basics such as Kenyon (out of date but still good, Datig and Jones (all very old, but still pertinent) - you can get these fairly cheap, and then specialty books on certain subjects, Krieghoff by Gibson, Simson, police books, etc. The ones that Marc said above are very good. Notice a few of the cheaper ones, Reese, Davis were not mentioned. They are fine, but not my first choices.