The X Files Origins: Agent of Chaos:

The X Files Origins: Agent of Chaos by: Kami Garcia
Fiction: Pop Culture, Mystery, YA, 307 pages
Physical Book Count:15
Book Count:44

This book is a story about Fox Mulder from when he was in high school.  This is after his sister was taken and so he already has his openness to the unexplained and mostly unused bed of the X-Files series and is working on being called only by his last name.  He is also in the D.C. area, as this is after his parents separated.  He is also already being watched by Cigarette Smoking Man.  In this book there are a series of kidnappings and ritual murders where the kidnappings remind him of the night Samantha was taken.  And so he sets out to solve the case so that he can at least save the life of another 8 year old girl.  He gets help from Phoebe, his best friend from his hometown, Gimble, his best friend in the DC area (who is all about the new dungeons and dragons game and goes by his character's name, his real name is Gary,) and The Major, Gimble's father who became obsessed with conspiracy theories and the book "Stormbringer" after his wife's death in a car accident.   Of course we get backstory for things in the series, like the saying "The True is Out There." But like filler arcs in anime it is a zero sum game, so whenever Mulder is in a life or death situation you know he will make make it out and they can't add any backstory for something not in the show, so you are also pretty sure that his friends aren't going to be murdered in front of him or he runs into Cigarette Smoking Man or any other life changing event that doesn't fit with his character in the show.