Conrad's Fate isn't among the Diana Wynne Jones books I've reread frequently. Partly, it's because it's not a favorite, and partly because I read it much later than the first set of Chrestomanci books, so it didn't seem to quite fit. However, on this reread, I'm more lenient towards it than on my initial reading. Specifically, I think Christopher (who is charming and arrogant) is very well-portrayed as a secondary character. Although Millie and the others could be more vivid. Stallery Castle and its probability shifts are a great concept, and I would have loved to see even more playfulness with them.
2005 Read:
Conrad Tesdinic resides in the town of Stallchester, which experiences minor reality shifts. Items aren't where people left them, shirts change color, or book titles alter. According to Conrad's Uncle Alfred, this is because the people in Stallery Castle are "playing the probabilities" to manipulate the stock market, much to Alfred's miserly envy. When Conrad is twelve, his uncle sends him to Stallery Castle to kill someone and expiate the bad karma Alfred claims Conrad acquired in a previous life. On the way to the castle, Conrad meets a well-dressed and somewhat arrogant fifteen-year-old named Christopher. Christopher joins him in training as a servant but is actually an enchanter from another world searching for a lost friend. If you've read the previous Chrestomanci books, you'll recognize the future Chrestomanci. He's a few years older than in The Lives of Christopher Chant and considerably younger than in Charmed Life.
You can read Conrad's Fate without having read any of the previous Chrestomanci books as it's not a direct sequel. However, I'm not sure if I'd recommend it. The supporting characters all seem somewhat sketchy and underdeveloped, but for Christopher, you can at least fill in more details from earlier books.
This is much better than The Merlin Conspiracy, but still not as good as Jones at her best. Conrad is likable, and the details of his learning to be a servant and navigate the Stallery social hierarchy are vivid and convincing. However, the book doesn't quite come together right. It's clear very early on how Conrad is being manipulated and what he needs to learn, but he doesn't actually learn it from the plot; he learns it from being told. The plot and character development each work okay on their own, but they don't reinforce each other. Also, Conrad would have felt more rounded if he'd interacted more extensively with more than one person. The denouement is also one of the most rushed and haphazard I've seen. Jones often has trouble with this, but I think I'd prefer the abruptness of Fire and Hemlock or A Tale of Time City to the very eighteenth-century epilogue we get, which simply doesn't fit with the way the rest of the story is told.