Almost everything you ever wanted to know about the series can be found here. Character profiles, volume summmaries (including publishing information and cover/poster scans, both the TP and original Japanese versions), differences between the English and Japanese versions, and other random stuff that I might throw in here.
About the Series
Author: Mineko Ohkami
Publisher: Shinshokan
Serialized in: WINGS
First published: 1990
Finished: June 2007, last volume published in August 2007
Genre: Fantasy/Comedy/Action
Licensed by: Tokyopop (2002-2008)
Dragon Knights was first published back in 1990 and finished at 26 volumes in August 2007, the last chapter publishing in the June 2007 issue of WINGS. It was serialized in the magazing WINGS, which is published by Shinshokan, who also published the tankubon and consequent bunko versions. It was later licensed and brought over to the United States in April 2002 by Tokyopop as one of the first few series to be printed in the smaller Right-to-Left format. It finally finished publishing in June 2008 (after a ridiculous six month waiting period despite the fact the series ended a full year earlier).
The series itself is rather in depth and can be complicated and hard to read in just one sitting. As there are several details and facts that aren't revealed until late in the series, rereading the whole series more than once is almost required to get a better grasp of understanding everything that is going on. Though there are three main characters - the Dragon Knights Rath, Thatz, and Rune - there is still a whole slew of characters that are involved. The fact that this is also in a fantasy setting opens up a lot of creative license on Ohkami's part, so a suspension of disbelief is highly recommeded. If you try to put too much real world logic into the world, you'll get yourself more confused >_> I don't really have a problem so I never noticed it until I hear some other people get confused and whatnot.
Brief summary provided by Aquajogger: The three Dragon Knights, Rath, Rune, and Thatz returned to the Dragon Castle with the defeated Demon Lord Nadil’s head. They were soon sent out on different missions, however: Thatz was partnered up with his old rival Kitchel to find the Three Treasures; Rune went to discover the reason behind the disappearance of the Elves and to free Tintlet, his wife, from her enchanted sleep; and Rath, after fighting the demon Bierrez who had attacked the castle while the other two were already gone, snuck out with the fortuneteller and Wind Demon Cesia to find the Wind Staff and the Wind Dragon. Instead of the dragon, they found a young man named Gil and unforeseen trouble. The Knights returned to the castle after their separate quests, unaware of the attack by the demons that was about to unfold. Tragedy struck the Dragon Castle and the Knights set off for Kainaldia once again, although this time on a rescue mission. After fighting the demon Rim Kaana who had been sent to stop them and Sabel who had his own ulterior motives, they reached Nadil’s castle. There they met with old allies and some unexpected new ones, rescued Cesia from her imprisonment, and entered into the climactic final battle against the Demon Lord…
Also, despite the series being finished, there are still many questions and characters unanswered. While some could be deduced after a while, there is hope that some of these answers will be revealed, or at least more concretely, in Ohkami's possible sequel to the series. She is currently working on a new series, Strawberry Palace, and according to her blog, she is considering continuing working on Lumen Lunae and Dragon Knights. We can only cross our fingers and hope, and in the mean time I'll try and gather as much information and explanation as I can to help repress and remove any confusion xD