Well. I don’t know how responsive he is, but he does have a Facebook author profile, so you could try asking if he would share the origins with you?
I’m sure that he spent a lot of time researching it prior to releasing the book. I was going to share the link but I am not sure it fits within the forum rules, but if you search Tristan Whitespire in google, it comes up right after his amazon link. Sometimes authors will share how they can be connected with on amazon, but hasn’t, and doesn’t appear to have any sort of webpage of his own.
I doubled checked my book and he only links to his other books in it, so I’m assuming he is not hosting a webpage of any sort at this time. So it seems the only way you could try to find out where he found the origins, would be through messaging or posting on his Facebook author page.
Granted you could try to googling the words spelled correctly, but I have a hard time working backwards to determine what the original spellings were for the ones I don’t see often, since he only gives his pronunciations.
I tried searching Toviel and ended up with all sorts of poems and things, but not really any reference to the angel’s original appearance in literature.
I tried searching YICH-EH-AK-EV-CAH-HA and I could only find it in another ritual, for an invocation of Bune, and a ritual written in Chinese using the same exact words and Hebrew letters that he uses in his books- but on like playing card images. So I’m not able to determine where he sourced it from.
I do keep seeing someone named Tristan Butler come up with works very similar to his, that has a Patreon, but I don’t know if it’s Whitespire or not. It could be Whitespire is a pename and Butler is the real person.
Edit: Actually looking at the way this Butler guy speaks and the topics he has on Patreon, I’m almost certain that Whitseprie is his penname. He talks about the genies of the hours and some things he was releasing there instead of into a book and yeah. It seems you could probably reach him via that as well.