First, ignore the incredibly well designed, semi-holographic cover. It's beautifully set, with a calming blue colour scheme and an intricately layered series of cogs behind an intriguing illustration of a clockwork bird. The cover design seems serene, which perhaps makes it very badly designed, because you really should ignore the cover. The Watcher in the Shadows is not a serene book, nor is it calming, nor is it, really, all that intricate. That cover lulls you into a false sense of security and then BLAM! Possessed puppets, shadow monsters, murder, and literal devilish pacts.
I find it remarkable that The Watcher in the Shadows is aimed at pre-teens. I'm not great with supernatural horror, it tends to creep under my skin far more than a rational adult should allow, but I think I would have been scarred if I'd read this book at the age of eleven or twelve. Which isn't to say that TWitS (heh, twits) is a bad book: it isn't. Nor is the text beyond the reading skills of a pre-teen: again, it isn't. But being a well-written book, whose characters you quickly come to empathise with, the constant 180 pivots from happy country life to sinister murder fest hit hard and deep. It is an unsettling book.
The nature of the horror is, I think, why it got to me quite so much. There isn't any gore or discomfort here, the narrative and descriptions are distinctly PG, but that only serves to amp up the chills. The least scary part of the book (opening, non-supernaturally influenced introductory chapters aside) is the sequence where Ismael and Irene are actively being chased by the killer angel. The angel is described in a manner which would be terrifying to see, but the very fact it can be described means it can be reasoned. Not reasoned with, but simply understood. By contrast, the points which lingered in my mind, steering my actions away from reading more on several occasions, those were when the evil was merely hinted at, a formless malice whispering on the wind. It's a commonly understood paradox of horror that the moment you see the monster it loses some of its power to terrify. Simply put, what you can imagine will always be more personally terrifying then what someone else can, because it ties into your own fears and insecurities. It's why creatures like Doctor Who's weeping angels or the original xenomorph in the first Alien film work so well: they spend most of their time simply being a threat, not an active and visible danger. The suspense is what gets to you and TWitS is full of suspense.
Although, it's also not that long. That's definitely a compliment on Zafon's writing, as the narrative feels much longer than the page count would allow. Despite the events themselves only taking place over a very short period, the character's are developed extremely well. That makes the book an absolute joy to read and helped pull me through the more action-oriented sections. In fact, I'd argue that the characters are the second reason why aiming this book at pre-teens is just wrong. The way they are written and built-up is subtle, intricate and relies on a level of maturity I feel the target audience would lack; I certainly would have at that age. Again, for a mid-teen reader it's probably spot-on, not a pre-teen.
That said, there are elements which did feel a little immature. The evil is sinister and the main characters are well fleshed out, but the story doesn't so much as lead you to conclusions as drag you there and slam them in your face. It manages suspense well, but the "clues" given as to the true nature of the evil, and of Lazarus himself, are extremely obvious. As too are the motivations of the antagonist. There's little depth to these elements of the plot: an abused, effectively orphaned child flees into the waiting arms of the Devil. It's the classic Monkey's Paw tale, only with fewer wishes and more manipulation, meaning that Lazarus himself is purely a victim, absolved of all real guilt. There's also plenty left unexplained. Based both on the story of the perfect clockmaker that Lazarus tells and the odd scene where the shadow consumes several weaker, pleading shadows of other people, it would seem that the Devil's shadow technique is not as unique to Lazarus as his backstory would imply. If that is the case, though, why have his mother so utterly terrified of the boy's shadow? There's also the idea that Lazarus could possibly forget his deal, or even doubt it's otherworldly nature. He's a man who doesn't have a shadow! That ought to have been noticed constantly throughout his life, even if only by him, a permanent reminder of the pact he agreed to. Certainly, you would expect him to have kept the shadow (encased in glass) locked away! Finally, there's the village itself. We're told that they had a string of murders in the past, which are assumed to have been the shadow. That questions why the shadow doesn't go on a killing spree upon it's second release, but it also raises problems with the competency of the local police and general population. A slightly odd man moves into a mansion on the edge of the village, within a sinister forest, and a string of murders shortly follows; his own wife is then struck down by an unknown illness and never seen again; years later, in the same forest and not far from the man's house, his own maid is brutally murdered. There appears to be a clear trend here, but the police never so much as appear to suspect Lazarus.
Luckily, it does avoid the slightly creepy trope of marrying of the child protagonists. There's something so much more wholesome about a first-love that isn't a last love, but simply fondly remembered, and the closing chapter does a great job of fleshing out both the surrounding narrative and the characters themselves. It's a neat ending which gathers all the threads together (well, most of them) and ties them off whilst leaving the conclusion naturally open-ended. The feeling is that the story being told is done, but that it only comprises a small part in the character's own lives. That's a nice note not often found in teen literature and ensures that, despite its flaws, TWitS will be a book I remember fondly.
tl;dr: