As far as what to *start* with, The Fainting Phenomenon by Dr. Blair Grubb is written to be accessible to laypersons. None of the actual medical textbooks are easy reads without some foundation in biology/physiology/anatomy/etc. ... keeping a basic "intro to neuroscience" textbook handy for reference (can be gotten cheaply at used book stores) is helpful.
Many of the titles that I've found useful to date can be found among the listed writings by the authors on this page:
You can find a great deal of the best writings that have been done specifically on OI disorders of dysautonomia (such as POTS) by looking for these authors/doctors: Phillip Low, Christopher Matthias, Blair Grubb, David Streeten, Julian Stewart, David Robertson, and Fetnat Fouad-Tarazi. There are TONS of autonomic texts on amazon.com so I suggest looking on there to get ideas because many have the preview feature that allows you to view the table of contents and read a few preview pages to get an idea of what the text is like so you know if it's what you want. Then you can narrow down a short list of what you'd like to pursue from the library. Keywords like "syncope" and "autonomic" are probably most useful, rather than looking for general neuroscience books which will mostly talk about the properly functioning ANS, rather than delving into the specifics of clinical autonomic practice (i.e. autonomic dysfunction). Happy reading!!