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.
I'm fine with books being canon (I'm a HUGE supporter of the Star Wars EU, the EU is why I love the Star Wars Uiverse). I'm sure the B5 books are good. They're also fairly expensive to get anymore, outside of getting lucky at a thrift shop (I saw a B5 book for 50 cents once, it was before I got into B5 so I didn't buy it, I still regret the missed opportunity
). The technomage stuff does sound interesting, if a bit unexpected.