Ok well i'm very tempted to write some Babylon 5 fanfic piece's now i've seen all the series' but i was wondering how useful would the rpg books be to me as i'm not that great on fluff about Babylon 5.
All the fluff you could ever need, from episodes to race background, from equipment names to planetary descriptions, from jumproute network to political entities, from sample NPC's (which you can either poach to use as "guest stars" in your fiction, or connect with by using their names and having a relative of them show up in your piece - in any case it adds more "depth" to your BG) to pictures of ships to describe.
And besides all this, there's a in-depth look behind the scenes, from the political relations between the Centauri High Noble Houses to black project hiddin in Earthforce, from the skeletons in the Minbaris closets to the way the Drakh Entire is structured, from the trading laws of the Vree the truth behind "The Hand", it's all there.
So, if you want to do some fanfic, do the research from Mongoose supplements! (And AoG supplements, and CEE supplements, but those are no longer in production and thus hard to find... in any case, Mongoose stuff contains a lot of what they did, and more besides)
It's what I do whenever I need to put together a piece of fluff for my RPG campaign or other projects...