Book 1 of The Chronicles of Narnia.
The reasons why I dislike the Narnia books are many and varied. I actually loved these books as a child, and devoured them repeatedly when I was ten or eleven or so, which probably accounts for some of the vehemence with which I now dislike them: I feel somehow betrayed. Of course, as any adult or perceptive person knows, the Narnia books are blatant Christian allegory written by a Christian apologist. That alone would of course bias me against them, and may again contribute to the vehemence: I loved these books as a child; as an adult I recognise that they were an attempt to mould and manipulate me. This may sound a bit strong, since there’s no secret about their being apologetic in nature, but they are geared toward children who may be unequipped to understand their nature—as I was.
Still, not all of my dislike has to do with religion or ideology; I think the books are simply bad qua books. The most glaring problem is that the apparent protagonists are not actually protagonists at all. Protagonists do something (cf. the etymology, agonist, Greek agōnistés, &c.). The children in the Narnia books appear to do things, but they are all ultimately irrelevant. One really should see it coming, but there it is: Every crisis, every serious issue in all the Narnia books is resolved by a major, in fact a literal deus ex machina. There’s a reason why the deus ex machina is a faux pas in literature: It removes tension. When not merely one, but all major problems are so resolved—
Additionally, I share an objection to Lewis’s style first expressed by his friend J.R.R. Tolkien: The piecemeal composition of Lewis’s mythology is æsthetically unpleasing. A faun here, a Jesus-lion there, and a Santa Claus stopping by for a brief halloa—it seems a hodgepodge and it does not create any sense of authenticity.
So: An underhanded message, delivered in a manner that sucks out all the tension, placed in a clumsily constructed world. Colour me unimpressed.