For maximum impact, I'd suggest a slight twist on the standard order. Read 'The Silent Corner' first, obviously. Then, if you can, take a small break before diving into 'The Whispering Room'. Let the haunting ending of book one sit with you. The sequels come at you fast and furious, and it's easy to get numbed by the constant action. Savoring the dread Koontz builds in that first installment makes the escalating chaos of the later books hit harder. The series is a marathon, not a sprint, despite its pacing.
It's a pretty straightforward timeline if you go by publication order, which matches the internal chronology. The first book is 'The Silent Corner', then 'The Whispering Room', 'The Crooked Staircase', 'The Forbidden Door', and finally 'The Night Window'. That's the complete five-book series.