Yes, transmedia storytelling is still a buzzword-ish sort of thing. Idealistic creatives strive for it, at times, while producers/anyone dealing with the numbers of it all generally find it worthless.
It is not done well with WoW, and probably will never be done well at all for any IP, in no small part due to the fuzzy line between “telling part of the story with a novel” and “let’s merchandise the hell out of this brand!”
Personally, I believe that in order for this to work the secondary forms of media have to be part of your customer base’s preferred forms of media e.g. if the majority of players were, you know, readers, then you MIGHT be on to something with novels as part of the transmedia spread.
If it were a hardcore RPG, where RPers where the overwhelming majority, then books might work. Still, you’d need to bundle them with the expansion – the base expansion – and/or make them available in some cheap, digital format e.g. free e-books.
Because otherwise, you’re double dipping into licensing/merchandising and brand marketing, you’re trying to have your cake and eat it, too, and ffs that doesn’t really work in life.
Where have they done well with this? Probably the small comic-style animations and free digital comics. More of the playerbase will consume this content.
But here’s the thing… MMOs are persistent, so the use of additional media e.g. the Warbringers stuff should be sufficient during gaps where current content is getting to be old and overdone. You should not need, or use, a novel to tell part of this story.
Also, before you attempt transmedia maybe plan out a good story where you don’t have to retcon everything and destroy people’s confidence in terms of investing time into your brand.