Sources? Hmm. I have about a dozen books on the shelf and they invariably do not have the answer I need. There is a good one by Ball, and another by Frank de Haas (IIRC).
For starters:
- a matching German Mauser 98 (98K for Karbiner or short rifle vs Gewehr for rifle) has serial numbers all over. Matching is always desirable, but good examples are very scarce compared to mismatched.
- there are five pieces of information on a German 98K that you need to identify it from all its brothers. The year, the two or three letter code for the arsenal, the serial number, the serial number letter prefix, and the marking on the left sidewall.
- the completeness of the rifle, ie cleaning rod, front sight hood, and sling. You will always pay more to get a better example.
- be very careful with variations made in other countries. The most important two distinctions (there are many others) - large or small diameter receiver ring, which also changes the spacing on the takedown screws, and Mauser 98 vs Swedish 96 vs Yugoslav M48. These are not mutually compatible models. A 98 stock will not fit a 96 or an M48. Now, there are many many very interesting examples of Mausers made in other countries and learning about them is always a big part of the hobby.
FWIW, I have a Russian Capture 98k in the locker. Not for sale. I sold a Spanish FR7 La Coruna conversion to 7.62 NATO, and slightly regret my decision.