I second the advice on Poyer's Krag book. Another good book was written by Franklin Mallory, and it is still available through the Springfield Research Service.
Sporterized surplus Krags from Bannerman were very popular mail-order items during the 1920s and 1930s, and there are still a lot of Bannerman sporters floating around that are being sold as 1898 "carbines". They might be worth a bit more than the average chop-job, but they are not original.
In terms of numbers, the Model 1898 rifle was the most common, followed by the Model 1896, and then the 1892. Original 1892 rifles are pretty much unheard of. Original carbines from any year are somewhat rare and bring a premium.
Real Krag carbines have a saddle ring, and have the front sight post brazed to the barrel. They are hard to find. If the gun you are looking at has an M1903 front sight band, that is a dead giveaway that it's a Bannerman sporter. Also a blued receiver is a giveaway; the original Krag receivers were gray case-hardened.
Also look for a JSA stock cartouche with date equal to or later than the model year on the receiver. Original rear sights were changed several times, they can be just about anything. Carbine rear sights look the same, but they were calibrated to longer ranges than rifle sights due to the shorter sight radius.
Good Krags are hard to find, but worth looking for. Factory ammo is still loaded by Winchester and Remington, but it is a seasonal item. The safety issues with early M1903s don't really apply to Krags. The receiver alloy is the same, but Krags have a fully supported chamber and operate at much lower pressures. Shoot and enjoy.
ETA: The #1 giveaway on Bannerman sporters is the square wooden plug under the barrel in the forend of the stock. A square plug was used to fill the channel in the full-length rifle stock after it was cut down to carbine length. You may have to look close to see this plug.