Last night Tad Williams was in London to do a signing at Forbidden Planet and also to take part in an interview and Q&A session. I had the pleasure of meeting him for the first time and it turns out he's a really friendly and nice guy.
New information from the signing: Tad confirmed that Empire of Grass, the second volume of The Last King of Osten Ard, is complete but needs to be edited. The publishers are mulling over when to release it: if they go for an early release (presumably in 2018), Tad will immediately start writing the concluding book of the trilogy, The Navigator's Children. If they decide to hold off until later, Tad will write the second short prequel novel, The Shadow of Things to Come, first. Unlike The Heart of What Was Lost, which was a bridging novel between the two trilogies, Shadow is a completely self-contained story with no connections to the latest work; in fact, it's more about Sithi and Norn characters from the first trilogy in the heyday of Sithi civilisation (Sithilisation?), so can come out before or after the trilogy is done.
Otherland remains under a film option but Memory, Sorrow and Thorn is not under any option at the moment, despite some vague interest from time to time.
Tad prefers writing his shorter novels like the Bobby Dollar books, which only took four to five months each to write rather than two to three years like his big epics. We may see more shorter books from him in the future.
When asked which of his fictional worlds he'd like to live in, he replied "The one with the best toilet facilities".
Video blogger Kitty G recorded the interview so keep an eye on her YouTube channel to see when it goes up.