When I read "The Thief of Always", it occurred to me that it was not a novel as much as it was a fable, a distinction that seems to have occurred to others, as see the write-up above, as well as its description on tvtropes and the normally bland wikipedia.
This was Clive Barker's first work for young adults, which meant that the horror factor was dialed down a bit, although since this is Clive Barker, it is still scary and disturbing.
There were several versions of this book published, one with Barker's own illustrations, and one as a text-only version. I read the text version, which may mean I didn't get the full impact of the book.
The plot of the book follows one young Harvey Swick, a typical bored preteen from an unspecified time in the 20th Century, who gets an invitation to go the Holiday House, a place where the high points of childhood occur everyday. Every day has a halloween and a Christmas, Harvey finds two other children there, Wendell and Lulu. While at first seeming an ideal break from his boring life, Harvey soon finds out that the house is not all fun. In fact, it is downright sinister, and young Harvey Swick must gather his courage and his wits to return back to his normal, boring life.
Clive Barker is somewhat of a writer's writer. As I said above, this book is a fable, more than it is a novel. Barker takes the entire story of temptation and identity and distills it down to its essence, in a way that many writers would find enviable. It is the same basic story that Neil Gaiman would turn into such a success with Coraline. But while The Thief of Always has its following, it didn't seem to break out into being regarded as a classic. As with much else of Barker's writing, it is a finely crafted tale that tells an interesting, important story, but seems to leave out some element of characterization that could have made it a phenomenon.