The Arduin II (Compleat Arduin) is the 2 books that were originally called Compleat Arduin (it was hardly complete but was a noble effort without a doubt (I can only imagine the headaches on putting it together). Anyway Arduin II holds the vast majority of the spells, monsters and character classes that were published in 1-9.
The differences lie here:
1. Arduin II uses its own combat system (d100) (ag 1-3 used d20) and pretty much a different spell casting system.
2. Arduin II does not have any of the world information which was prevalent in the 1-9 books. The World Book of Khaas contians a lot of the worldly material that was in 1-9 and greatly expands upon it and organizes it in a massive undertaking.
3. Arduin II is significantly more organized, but does not have a huge following. The actual book 1 of Arduin II (the first half) is a free download on the www.empcho.com
site if you would like to have a look at it. Thi sshould contain the contents as well.
4. Arduin II had a little bit more material tossed in from whence it was calle dthe "compleat Arduin."
5. The grimoire volumes 1-9 are being kept in print for the following reasons:
5a. They sell (what can I say with the limted products we have available the trilogy even in the printed format it is in (as close to original including bad print) sells. It is our #1 seller.
5b. Nostalgia for anyone looking for them
5c. We cleaned them up and reformated 4-9 (soon to be 1-3 if I can finish em) into a more readble clean looking copy that is priced affordably and worthy of just collecting and mining for ideas or using in any game or writing technique.
5d. We have not completed Arduin Eternal so we dont have a system that we want to tie everything onto for the future as of yet.
In my opinion - buy em all! were a small company LOL. But seriously it really depends on what you are looking for. If you are looking for world material and setting information, then I would say the World guide, followed by the Grimoire 1-9. If you are looking for a more complete system because you want to try something new then I would suggest Arduin II, but maybe try the download first just to see if you will like it (quite deadly). I would not personally recommend Arduin II if you are looking for tons of worldly information, there is some in there but not like in the other books.
Half the battle was to come up with a system that will encompass everything: System, World, Modules etc. that is one of the primary reasons we are developing Arduin Eternal. We could have used Arduin II but it just didnt fit the company at this time and never really had any following per say. We didnt want to rehash all the ocnfusion by trying to blend 1-9 with Arduin II and get it right. We just felt something more from scratch was appropriate.
I hope this helps.