Petroc Trelawney, Jenna's pet rock, shares its second name with a character in Harry Potter. Reportedly, he was named after a BBC broadcaster as a tribute.
Similarly, there's an unfortunate Mauve Shirt in Darke named Bertie Bott. After being asked, Angie Sage confirmed that this reference wasn't intentional, noting that there were other Botts in past books of the series.
The Other Darrin: For all voices and reading of the audiobooks after Magyk, which was read by Allan Corduner, who is known for numerous television roles, and has read audiobooks for several other popular titles. All later installments in the series were read by Gerard Doyle (reader of the audiobooks of Inheritance Cycle), and while he does a reasonable job, no attempt seems to have been made to emulate the voices used in the audiobook of Magyk, and some, such as that of Marcia, are very different.
Schedule Slip: There was an almost one-year delay in the publication of Darke, and the next book, Fyre, was only published in 2013. After that, however, things settled down, with the first book in the Sequel Series, TodHunter Moon: PathFinder, published the next year.
Shrug of God: Angie Sage is not always interested in explaining things.
Word of Gay: Angie Sage mentioned that Sam Heap and Wolf Boy are gay. During a Q and A with fans she explained it likely wouldn't be mentioned or shown due to the ages of the characters, but was interested in the possibility of showing it during a ten year time skip epilogue.
Which creates some mildly unsettling implications for why Wolf Boy was able to gain the job of Keeper, generally a female-only position.