The first time through this was the best book in the series so far for me after Vampire Science. This time through it runs a close third with Alien Bodies running a predictable close-first above the others. Where Alien Bodies spins mystery and misdirection across the whole meta-arc, Option Lock does the same thing on the scale of a single book; and succeeds (in my mind at least) in leaving the reader at least partly in the dark at all points even though, as Loyd Grossman would say, the clues are there…. And it ends with that amazingly apposite and ironic request quoted above.
Believable characterisation threads through this book, and it's far from short of its poignant moments. I remember wishing for some months that something like this would be written again, but we don't see the like before Fitz is introduced.