A pretty solid book, that could have been 5 stars but let down somewhat by the end. Some great ideas here, with silicon based lifeforms, and a bit of mystery surrounding what exactly happened to lead to the hand being on Earth, and who were the good guys / bad guys.
The one off characters were pretty good for the most part, and I was quite worried about some of the people working at the nuclear power plant, especially as it seemed like false hope to begin with, but the call home by the person in charge was quite a powerful scene I felt.
The Doctor was in pretty good form throughout, trying to avoid jumping to conclusions and letting it all play out, when others were quick to arrive at certainty.
The antagonist was a lot more interesting when female, but once became male, became very one dimensional, leading to a somewhat disappointing end after the strong build up up until then.
Some interesting moments for Sarah, but not sure it felt that strongly a reason for her to want to go through the motions of leaving at least, let alone ending up leaving - albeit not really her choice in the end though. Her actual leaving scene, while has a bit of bittersweetness to it, does feel overly abrupt, and not an ideal send off compared to what Jo Grant had at least.
On that point, given the differences during Sarah's time (initially her and the 3rd Doctor plus UNIT appearing in and out, then her and the 4th Doctor and Harry, then just her and the 4th Doctor), seemed more like with Jamie, where her leaving didn't feel as emotional, as it did with Ian and Barbara, or with Jo, though whether the scene itself also helped lead to that, I don't know.
I think first 3/4 of book were very strong, but it fell away at the end, leading to just 4 stars :)