At this point, the book is entirely fiction, even if you do believe the claims of the Lutz family.
At least, however, The Final Chapter makes for a better read than the second installment. The pacing is much better, and there are some genuinely eerie occurances. Still, Jones falters when he attempts to describe the physical forms of the demons. Rather than being an omnipresent, oppressive force of evil, it is reduced to a silly, slimy mess.
Of course, this was not ACTUALLY the final book in the series...