Sword in the age of chivalry?

Records of the medieval sword?

I really only want to buy one, but I know not which.

It depends on where your interests lie.

The Archeology of Weapons is an excellent rescource and covers swords from the bronze age up through the middle ages. If you're looking for a broader general reference this one is an excellent choice.

The Sword in the Age of Chivalry is specific to the middle ages so if your mainly after information on the knightly sword this is the one.

I personally consider Records of the Medieval Sword to be more of a companion to one of the other two rather than a stand alone work. It's primarily a photographic catalog of swords the Mr. Oakeshott found to be significant. A lot of the informational text from the other two is missing from this one.

Don't worry. You'll end up getting 'em all, anyway. : ) If not, then you should at least also get Oakeshott's European Weapons & Armour from the Renaissance to the Industrial Revolution. That way you'll also cover longswords, bastard swords, two-hand swords, complex hilt c&t swords and rapiers, military swords, basket hilts, etc. plus the armour styles that correspond to the evolution of the arms.

Yeah, what Sean said. ;)
I bought them all at once so I didn't have to worry about it.