There are a total of six novels featuring X-Files cases. Each novel stands alone from the others (each one is effectively a "Monster of the Week" story), and are presented here in the order the books were published. The first two X-Files novels, entitled Goblins and Whirlwind, were written by Charles L. Grant and published in 1994 and 1995, respectively.
The next three X-Files novels were written by Kevin J. Anderson. Entitled Ground Zero, Ruins, and Antibodies. Ground Zero was published in 1996; Ruins and Antibodies were both published in 1997.
The sixth X-Files novel is Skin, written by Ben Mezrich and published in 1998.