We here are huge fans of Diana Wynne Jones.
We admire the magical worlds she creates and her characters -- human, wizard, and fantastical -- captivate us. We find the plots of her stories unpredictable but plausible, at least in the magical environments in which they take place. One of the coolest things about her stories is that although the plot of each of her novels is really unique, characters and laws of magic overlap in intriguing ways in the many worlds described in her many stories.
We enjoyed reading Deep Secret, mostly because we became interested in Nick Mallory, who is a protagonist in another of Jones' many novels, The Merlin Conspiracy. However, it is not one of our favorite Diana Wynne Jones books.
For one thing, Deep Secret seems to mostly target adults, perhaps because it seems to be Diana Wynne Jones' tribute to science fiction conventions. The plot -- regarding a Magid (a powerful wizard whose undercover job is to keep magic under control in some sector of the multiverse) in search of a student -- is certainly compelling for certain young readers. But Jones unnecessarily throws in words (such as "orgy") that young readers are likely to ask their parents about.
Anyway, Nick is a nice, seemingly ordinary teenage boy with a witch (in all senses of that word) for a mother and a touching relationship with his ne'er-do-well cousin Maree. When my daughter and I first "met" him in The Merlin Conspiracy, he was looking for someone to train him to control his wizardly gifts. In Deep Secret, Nick seems not to be consciously aware that he needs training.
We enjoyed learning more about Nick and Maree and the Magid Rupert Venables and many magical creatures, including some fascinating centaurs and phantasmagorical chicks, but might not have found ourselves so riveted if we were not already familiar with many other stories in the Diana Wynne Jones opus.