Read the stickies here on the forum (under sub-forums, the ones at the top)
http://forum.lugerforum.com/showthread.php?t=6937
Old books with old info but still interesting - Datig, Jones, Kenyon
Imperial Lugers was 'essentially' replaced by Central Powers by Jan C Still - it is now sold out by Jan, but you can still find it.
Best advice given is keep calm - there are more lugers out there than you'd think, especially in the collecting world.
Ed