Getting a bit off topic, but the three trilogies (Centauri Prime, Telepath/Bester, and Technomages) are essentially considered cannon to the extent any tie in book series can be considered cannon for a show. All three were written based on basic story/plot outlines developed by JMS as part of his development of the overall Babylon 5 universe. And all 3 really are great and worth reading if you can find them. They fill in a lot of holes and finish a lot of stories, and even though they aren't written by JMS, they do it in the way JMS intended it to be done. Just some really great stuff.