This book is full of new rules and errata, as well as some more Rokugan background, short fiction and new army lists.
This one is pretty much the same, just for the forces of the really bad guys.
This pair was, I am guessing, the first published set and probably came with the original boxed set. I don't know, because I haven't been able to find an intact original boxed set. I actually already had a copy of the "Rulebook", but I figured since they came as a set and were really quite a bargain and you can't have too many copies of the rules around the table, why not?
This one is interesting in that it says it's the "Basic Rulebook" and it mentions the Daimyo edition, as well as sharing it's cover art; it also has a WotC logo on the back, next to the AEG logo. Sadly, it doesn't have an OA or a d20 or OGL or D&D logo anywhere; if it did that would make it kind of a defacto D&D miniature wargame by accident, just because it amuses me to think so.
Quick edit and editorial: I never noticed before, but the Daimyo Edition hardcover Clan War rulebook has the WotC logo on the back too, so do the 2nd edition Legend of Five Rings RPG Player's Guide and Game Master's Guide; additionally all of the Legend of Five Rings novels were published by WotC too. I am going to guess that AEG was owned by WotC for the entire run of 2nd edition L5R RPG and that is what led to the horrible decision to abandon Kara-Tur in 3e D&D in favor of the much more well developed Rokugan setting, despite the fact that 3e D&D as an engine wasn't really a great fit for the setting. Importing ideas from L5R into D&D didn't work all that well either, trust me, I ran a 3e OA campaign set in Kara-Tur and had to retrofit a lot of stuff; really devolving 3e to fit Kara-Tur more than evolving Kara-Tur to fit 3e. I hate having to rewrite classes and feats and monsters. That 3e Oriental Adventures book really vexed me. I had such high hopes for it, especially after OA was spurned during the 2nd edition era.