The X-Files: Ruins by Kevin J Anderson
Kevin J Anderson's second X-Files novel features Mulder and Scully in search of a young archaeologist at the site of a former Mayan civilisation. It feels like a book that will involve aliens, but in this case the book also includes ancient Mayan Gods and feathered serpents, so that it feels like an original idea not directly connected to the usual extra-terrestrials that appear in the X-Files mythology.
The first two thirds of the story built up the tension to an exciting climax, which is as heavy on politics as on science fiction elements, as other parties including the military and illegal artefact smugglers start causing trouble.
I quite enjoyed this book upon rereading, and enjoyed all of the callbacks to episodes of the TV show as well as references to the show's own backstory (mostly involving Mulder's sister). As with Anderson's previous book, the characterisation of Mulder and Scully felt spot on.
While some of the climactic scenes make me think now of elements from the first movie spin off, "Fight the Future" (released after this book was written), there was a great moment near the end that I had forgotten about. This is definitely one of my favourite X-Files novels.