It occurred to me today that essentially we have this great mystery of who is the Maker (ironic capitalisation) of the Gabriel Hounds. Then the great mystery is solved by Hollis mentioning the great mystery to Reg, who says 'oh, I know who it is'.
Then we have the tangentially relevant action sequence that Bill seems to think his novels need to have.
Then we have about 3 bizarre 'Jack's The Face Of Boe' style revelations.
And an Ekranoplan.
(also some weird retconning of characters. I'm sure, for example, that Voytek's English has got worse in the 9 or so years between Pattern Recognition and Zero History.)
So, in short, whilst Bill is a great prose stylist on a paragraph by paragraph level, and can capture a mood like no-one else, rather poor, certainly the weakest of the three, which is disappointing as I kind of excused Spook Country as being the weak book in the middle of the trilogy.