Actually living up to it's name, this is an almost scientific Vampire story, and where no explanation is provided one is almost tempted to believe that there might be one. Sam's clearly settling in to the Companion role quite well here, and a couple of years have passed from her point of view, although it would appear that she's not been in serious danger in that time.
Sam is doing the "should I stay, or should I go thing", which I feel is a little unwarrented this early in the series; although it works quite well in this story. Carolyn on the other hand is feeling allured by Sam's tourist lifestyle. Unsurprisingly neither decide to actually change anything (see, this is why I think it's too early!).
The Vampires themselves are well written, with convincing internal politics and at least stereotypes of personalities (and a convincing argument is made that they would be stereotypes. UNIT on the otherhand are presented as unremitting cynics, which is a good way of dealing with them, IMO.
I think this is probably my favourite of the early books.