“Serpentine” is a (really short) novella by Philip Pullman. It is set some time after the events described in the “His Dark Materials” trilogy, but before those in the “The Book Of Dust” trilogy. It is a richly-illustrated book containing a short tale that is unlikely to be of interest to anyone except for the fans of this series of books.
This novella is a tale describing the uneasy tension between Lyra and her daemon Pantalaimon. They had been separated towards the end of the first trilogy, which was quite painful for both, especially for Pantalaimon who felt betrayed by Lyra. However, they never got around to having a frank discussion about this uncomfortable topic, so the tension simmers and leads to a growing disaffection and consequent alienation. Can they overcome their inhibitions and become as close to each other as they were before?
The book, such as it is, seems to have been released to keep fans excited about the upcoming series of books in the new trilogy. There is not much substance to it. Even fans of the series might not find that much to be excited about in this particular book (I, for one, certainly did not). As far as I can tell, it does not add to the enjoyment of the second trilogy and does not contain anything particularly revealing or insightful.