IME, there is no one (or three) book(s) that is going to unlock the secret of a kid. For me, it was a cumulative understanding. I have read extensively (but that's a process issue for me in all things of interest). I have to say I have learned an IMMENSE amount on this board. I am entirely confident that we would not be where we are now without the wisdom of the parents here.
IMO, the Living with Intensity book is largely theoretical. Looking in my copy I see 4 potential chapters - 3, 5 8 and 9 - that offer more practical content.
I think the Webb book is a must-have in a parent's library if they're dealing with a gifted kid who's a little (or a lot) different.
I think The Spirited Child is a good place to start as she really breaks the information down into understandable sections. I also like her inventory/rating scale. I've heard Kurcinka speak and she's great. In fact, her book Kids, Parents and Power Struggles is a great read. It helps you to really look at what a child is trying to communicate, and I found it more "instructive" than Kohn or Neufeld.
IMO, you don't have to limit to looking at gifted literature - the intensity and sensitivity may be caused by giftedness, or co-occuring. If you accept and understand it as an essential part of who your child is, parenting books outside of gifted specialization have a lot to offer.
The other avenue is books dealing with sensory processing disorder. There is a gold mine of information in SPD about helping kids recenter and reorganize their systems when they're overwhelmed by stimuli (environment, emotion, internal etc.).
ETA: SENG gifted site has a lot of great articles.
Mom to a teenager and a middle schooler.