Just counted them on my shelf the other day—five books total in the Jane Hawk series. I think a lot of folks get tripped up because the first one, 'The Silent Corner,' reads like a complete thriller and you could be forgiven for thinking it's a standalone, but nope, it's a full five-part ride. The order is 'The Silent Corner,' then 'The Whispering Room,' 'The Crooked Staircase,' 'The Forbidden Door,' and finally 'The Night Window.'
Koontz wrapped it up neatly with that fifth book, no dangling threads, which I appreciated. The middle book, 'The Crooked Staircase,' is where the conspiracy really locks into place for me, with Jane fully becoming this mythic figure of resistance. If you're starting, commit to the whole set; the payoff in the final confrontation feels earned.