There's an awful lot of information out there just by Googling, but I found this YouTube video to be incredibly helpful on getting me started. This doctor is a well-respected expert. If you fast-forward to 40:36, you can skip to the most helpful information (IMO).

Or perhaps even better for less technical information, at 50:27 and forward, he talks about how MCAS presents itself, and what patients go through getting diagnosed.

Dr. Lawrence B. Afrin, MD, Immunology and Allergy: Mast Cell 11 - YouTube

Beyond just reading what Google will turn up, there are two private Facebook groups that have been extremely helpful and enlightening just reading the posts. It was in the histamine group that found people experiencing the bizarre neuro symptoms that no one here could relate to. They're beyond just the "usual" SFN symptoms.

Mast Cell Activation Syndrome Community Public Group | Facebook

Histamine Intolerance (Closed for Privacy) Public Group | Facebook

I should also mention how much better I feel just getting the widespread inflammation caused by high histamine under control. All the tendinitis, bursitis and general joint pain I had is reducing.