The Technomage trilogy is my favorite. It's not just an adjunct to a TV series, it's a grand, imaginative science-fiction epic in its own right that happens to be in the B5/
Crusade universe. That is, it ties in smoothly with elements from the shows, but tells a larger, independent story of its own that works as an SF novel trilogy even aside from its role as a TV tie-in.
The Psi Corps trilogy is flawed by the way it skips the actual Psi War. At the time, JMS had plans to depict that war in a later TV project, so he had Keyes avoid it. But since the war was never depicted, it just leaves a huge and arbitrary gap in the story and undermines the trilogy.
The rest of the books are pretty bad and were done without JMS' involvement.
No, they all had his involvement, just not to the degree he wanted, since his responsibilities on the show interfered. His intention was to supervise and outline all of the novels, and to proofread and approve them all to make sure everything was consistent with the show, but he underestimated how all-consuming the demands of producing a weekly TV series are, so his oversight of the Dell books suffered and a lot of inconsistencies and errors slipped through. (The Del Rey trilogies were able to be canonical because the show was off the air by then and he had time to oversee them more carefully.) Actually a few of them aren't bad at all, but they're variant interpretations that don't quite fit. JMS has said that he considers most of them to have some "canon value," i.e. they contain some elements that are "true" although some of the specifics are wrong. Although only
The Shadow Within and
To Dream in the City of Sorrows were counted as "true" canon and republished by Del Rey.
Of the other Dell novels, as far as I can recall, I think the most worthwhile one is
Clark's Law by Jim Mortimore. It's too big a story to fit into the canon, and kind of a variant take in some ways, but it's a very powerful tale. I also remember
Accusations by Lois Tilton and
Blood Oath by John Vornholt being pretty good, and reasonably close to canon.
But at all costs, avoid
The Touch of Your Shadow, The Whisper of Your Name by Neal Barrett, Jr. It's a total disaster and I'm amazed it ever got published. On top of all its other stupidities, it has scenes where people are watching cosmic events unfolding light-years away from them in real time, with the naked eye. (And despite its title, it has nothing to do with the Shadows.)
It's worth mentioning that, in contrast to the Dell novels, all of the
DC Comics are canonical. Overseeing comics is less of an investment of time than overseeing novels, so JMS was able to supervise and co-plot the comics more directly than the novels. The opening 4-parter, the first issue of which was scripted by JMS and the rest outlined by him, tells what happens to Sinclair between the end of the first season and
To Dream in the City of Sorrows, and its events are referenced in that novel. The second 4-parter is outlined by JMS and tells a story about a Shadow base on Mars which leads into "The Coming of Shadows" and events on the show. After that is an offbeat 2-parter by David Gerrold, and the final issue of the monthly run is a "Psi Corps And You!" promotional pamphlet (again from a JMS outline) -- canonical in that the pamphlet exists in-universe, though it contains many falsehoods. This was followed by a 3-part miniseries,
In Valen's Name, written by JMS and Peter David.