Spoilers, surrounded by * and written in white text, can be read by highlighting the relevant passage.
It took me quite a long time to get through this book. Despite its interesting premise involving creatures that come through cracks in time (I guess the Reapers were too destructive for Mann’s purposes), the story fell a little flat and the writing even seemed childish at times, clunky and unoriginal. There were two good supporting characters, the AI Arven and the retired old professor Angelchrist, which is quite a name, but these weren’t really enough to lift the story. The Eleventh Doctor, Amy and Rory were true to character, which unfortunately meant I found them just as annoying as in the television series. I really should have learned by now that I rarely enjoy stories from their era, but I optimistically keep trying to no avail. The story itself almost exclusively involved the characters running around London (future and past) away from the threat until they find a solution. The climax was acceptable, though there was a diversion that shouldn’t have worked given something that was established earlier in the book and a bit of a deus ex machina. *Moffat’s favourite ‘everybody lives’ rule also comes into play, because god forbid that stories end up having consequences for characters.*
One good element was the gratifying time paradox of the title. Not much time (ha) is spent on it, but it was an enjoyable moment nonetheless. The psychic abilities of the Squall also added an extra element of jeopardy, but this is mostly lost as George Mann seems to forget this later on in the novel.
Although parts of it were enjoyable and I found myself getting into it more as the book drew to a close, this is a very average Doctor Who story, even sub-par, and there are many far better ones you could spend your time reading (I recommend Human Nature, for one).