The discussion that Dwight pointed out probably has my same comments, but order of books to buy is below for me;
Stills books, depending on which era, for me, I have Weimar Lugers literally with me and I am traveling for work.
Kenyon's are fine, not enough info for me, but I like his.
I like Walters for his different view (he's a brit), tells more background info.
Jones and Datig are both good, but very dated, so there are wrong things in them.
Reese's books are fine, I have them, but he doesn't go into enough detail for me.
I eventually bought Gibson's Krieghoff book, it is great for the detail of him comparing the different type of guns. Don't have a krieg, so...
Guess, maybe I should write a book on Simson's, everything else has been done. Make note to myself, that everyone who has a Simson, please send me an e-mail so i can start researching them...
DougT is right, you can never have enough books, the comment about too many lugers, well, that is a given!