[racehorse]

Susan Howe's My Emily Dickenson is the only critical work I can think of which anybody really needs to read. An excerpt:
" Emily Dickinson took the scraps from the separate "higher" female education many bright women of her time were increasingly resenting, combined them with voracious and "unladylike" outside reading, and used the combination. She built a new poetic form from her fractured sense of being eternally on inteIlectual borders, where confident masculine voices buzzed an alluring and inaccessible discourse, backward through history into aboriginal anagogy. Pulling pieces of geometry, geology, alchemy, philosophy, politics, biography, biology, mythology, and philology from alien territory, a "sheltered" woman audaciously invented a new grammar grounded in humility and hesitation. HESITATE from the Latin, meaning to stick. Stammer. To hold back in doubt, have difficulty speaking. "He may pause but he must not hesitate"-Ruskin. Hesitation circled back and surrounded everyone in that confident age of aggressive industrial expansion and brutal Empire building. Hesitation and Separation. The Civil War had split American in two. He might pause, She hesitated. Sexual, racial, and geographical separation are at the heart of Definition."

A prude? Hell, she basically invented a grammar! Don't let anyone tell you Emily Dickenson isn't still experimental poetry.

Anyway, my favourite is the "M" dash.
I can't generate it on here but if you open WORD and type two words--and in between them type two dashes with no spaces then you've got it. It is fucking beautiful! So long....
I also love the Hamza in Arabic (ء) which can be a letter in itself, but it can also be used as a diacritic! It means "make a glottal stop" which is that sound that replaces the "tt"in Cockney English "butter", or the throat sound in "uh oh".