There are so many things / items / tid - bits of information that we must observe, identify and put into logical sense and or sequence. NFPA 921 is one of those "items" that can be used for you or against you. I agree that it is a guide and all other books that have been published are guides that can be "referenced" to in order to not only make a definitive determination but also an "accurate" determination, even if after hours and hours of investigation and research your conclusion is UNDETERMINED. That is your honest answer. Going back to 921...it is there...we should all be aware of what is in it. Realize that it can help and can hurt your case. None the less, be aware. I don't advocate any one book or reference as authoritative. I've been in the business for 15 years on the public side and almooooooost 2 years on the private. So I have been around for little while and if you ever thumb through my personal books of 921 or Dehaans you will find my grubby little sooty fingerprints on most of the pages. Bottom line, be aware of your surrounding environment and identify the things that can hurt or help.