The book you mentioned by Aaron Davis, The Standard Catalog.... is probably the best out there right now. It superceeded the older books by folks such as Costanzo, Datig and Kenyon. They still have useful information, but they also have a few problems and generally are pretty pricy as a result of having become collector's items on their own. The book by Davis was supposed to have been reprinted by someone and I have seen a couple of copies floating around recently, but I think that they were pirated copies. They had spiral bindings and looked like they just came out of somebody's copy shop. There is also another version or versions being put out by a company called Duckett Publishing Company and seem to be with Davis' approval. The are under a slightly different title -- one is The Handbook to the Catalog of Lugers... and the other is Catalog of Lugers... (there is no "s" on the word Luger in the original book by Davis). In any event, you can find them on ABE Books website. I have not purchased either one, so tell us what you find.
The books by Walter are ok, and the books by Still are great but generally focused on a specific period and, once again, they tend to be very pricy. Spend your money on Davis' book first. Then... if you get to the point that money is no object, the book by Joachim Gortz & Dr. Geoffrey Sturgess, a three volume set, is the state of the art, but there are at least three editions (2010 and 2012, 2 in English, 1 in German) of that set and right now I can't remember which one is the best. If you're really going "to get into Lugers," you might want to focus on a particular model or era. You'll also want to put the websites for Simpson Ltd and Jan Still's Luger Board along with a few others on your phone. Best of luck.