Answer:
found at the back of the book.
I am commonly abbreviated to H
I can be found after you remove 6 letters from BSAINXLEATNTEARS
People need me yet they give me away every day.
Adored by few, feared and hated by many. Mistress of the entire universal reason, master in the art of numbers. Some may have solved many of my mysteries, but there are still many of them to find. What am I?
Almost everyone needs it, asks for it, gives it. But almost nobody takes it.