I'm not an expert on the markings of the P-38, but here's some info on yours.
As above it's a P-38 pistol, not a Luger. The Luger is a totally different gun.
The slide markings of "P-38" "AC" and 5528 f mean:
This is a German military P-38 pistol.
It was made by Walther, who from the early war to the end was assigned the code "AC".
This was designed to disguise from the Allies just which factory had made the gun.
"43" indicates the year made, 1943.
The serial number is 5528 f. NOTE that the small case "f" is PART of the serial number and is very important.
The grips are made of brown Bakelite.
This sounds like a more or less standard Walther-produced 1943 P-38, made in the "f' block.
About the serial number: The P-38 was made by three makers during the war: Walther, Mauser, and Spreewerke.
Walther and Mauser assigned 4 digit serial numbers running from 0001 to 9999.
The way this system worked is, starting on January 1st, the first gun off the line would have serial number 0001.
When the serial numbers reached 9999, the sequence would start over at 0001 with an "a" added on the end. This block of guns would have numbers running from 0001a, to 9999a.
At that point, the next block would start at 0001b, and run to 9999b, and so on.
This is why the small case letter after the serial number is so important.
Due to the numbering system, during 1943 alone, between Walther and Mauser, there could be literally dozens of P-38 pistols with serial number 5528.
How they are separated is by maker, year, and the small case letter block codes.
Value on WWII P-38 pistols has been steadily climbing over the past years.
Actual value depends on several factors:
Actual condition.
Maker. Walther (AC) guns are often more valuable.
Whether the gun is import marked.
Whether the gun's serial numbers all match. The slide, frame, barrel, and barrel locking block will all be numbered.
Whether the gun has ever been refinished.
The year made. As the war went on the quality of the guns deteriorated. The earlier the gun, the more it's worth.
A good way to fix a price would be to look at finished auctions on the gun auction sites:
http://www.gunbroker.com/
http://www.auctionarms.com/search/search.aspx
http://www.gunsamerica.com/