If you want to go the opposite direction, Hamilton's "Elements of Quaternions" is long out of copyright: https://archive.org/details/elementsquaterni00hamirich/

If you want to go the opposite direction, Hamilton's "Elements of Quaternions" is long out of copyright: https://archive.org/details/elementsquaterni00hamirich/page/n0 https://archive.org/details/elementsquaterni00hamirich/page/... (I doubt most people get past the impressively long table of contents. :)