Mark (who is probably the first person to link to this blog, if not the first person to read it regularly--hi Mark!) asked if there will definitely be a fifth Turing book.
Without giving any spoilers here, it's fair to say that in addition to the main plots of the second and third Turing books, which are solved by the end, there's also a larger plot arc that remains unresolved and carries over from book to book. And (small preview), that larger plot arc takes a twist in the fourth book, but remains open. Will I wrap that up eventually? Yes.
But I'm not sure how quickly. Writing a series is, in many ways, a collaborative process. Yes, it's ultimately my book, but there are a lot of people who influence the way I think about it. So it depends on what my editor wants. What my trusted critiquers think. What my readers tell me. Is it time to wrap up that arc and move to another? Not yet, as far as the fourth book. Next time out, who knows? I'm working on a Meg book now, and marinating future Turing ideas.
And then there's the ultimate uncertainty of the publishing business. Even if you have a contract, publishers can decide, for a variety of business reasons, not to bring the books out. And I think every series has a natural life--I've heard even Sue Grafton plans to stop at Z is for...whatever. How long will this series continue? Who knows? (Longer, obviously, if as many people as possible vote with their wallets by buying it, and talk it up to other people who might like it!)
But I've promised a number of people--myself included--that if the series in its present form ends before I wrap up that large ongoing plot arc, I will find a way to complete the arc. How? Dunno; depends on what's happening at the time. I just know that I won't be happy if I leave the characters hanging.