I find "The Luger Book" by John Walter to be very informative and useful. It is in an encylcopedia format, with topics listed from A-to-Z. Out-of-print...but can be found on e-Bay and gun bookstores from time to time such as Ray Riling Arms Books :
http://www.rayrilingarmsbooks.com/cgi-bin/rrb455.cgi
Do not confuse "The Luger Book" with other books by Walter titled "Luger" and "The Luger Story".