*sigh* I really wanted to really like this book. And not just wanted, I expected I would — quite apart from being a fan of Shamus works in general, I positively devoured "Free Radical" and "How I Learned", and enjoyed them immensely.
His writing has always been short without being abrupt, flowing without being hard to follow and thorough without being needlessly verbose.
So it is with some trepidation that I have to come out and say that The Witch Watch left me with an impression of being more like a manuscript than an actual book. Or a children's book, maybe.
But let's get some of the good stuff down first, so that it doesn't seem like I didn't like the book (I did, just not as much as I had hoped). The story is interesting and gets going from page one — no enduring a long introduction before you "get to the good parts". The alternate history London/world is very well thought out, and shows itself unobtrusively, without long-winded exposition (a skillful application of the "show, don't tell" principle that's so important in Shamus' professional background — videogames). A strong, independant central female character is a bonus as well, and even more so because she's a reasonable strong, independent female character. Too many books have the tendency to equate independence and strength of will with stubborn recklessness.
So it's definitely worth a read, and if you're getting the $5 digital version, the money won't be wasted. That said, it's not all wine & roses...
Most of the dialogues seem awkward and stilted, and maybe as a consequence, almost none of the characters (with the exception of Gilbert's mother, interestingly) seemed not entirely fleshed out, like they were stuck partway between cardboard cutout story characters and real people. It gets better with time, but the feeling of them not quite being there never disappears entirely. Also, many instances of the spoken dialogue felt lacking an emotional "wrapping", i.e., an accompanying mention of the speaker's facial expression or body language.
The same "only sort of there" problem seems to afflict the world of the Witch Watch. For about the first two thirds of the book I was left with the feeling of watching a low-budget theater play, instead of experiencing part of a living, breathing world. I'm no literary critic so I can't readily point out the reasons for this, but until near the end, the world around the characters never seems to be very real, despite the obvious work that has gone into the world-building, from overall sociopolitical situation down to the scientific details of how the magic works. Only at the end, when more and more people get involved, does it all start to really come together and bring tho world to life.
There is one thing that might have affected my perception of the book, though. I was reading (listening to the audio book version of) Diana Gabaldon's The Fiery Cross at the same time as reading The Witch Watch, so maybe some unconscious comparisons did take place. The Fiery Cross is much larger work, written with much more detail, not to mention a fifth book in a series with four books worth of world-and-character-building behind it. If compared side-by-side, The Witch Watch could unfairly appear somewhat rushed and simplified.
In summary, I think it would make a great children's book, especially for kids who are into action-packed adventure stories, and at $5 it's well worth the price. Just don't get into it with high expectations.