So, for the Iesnium ore, we have an information tab in JEI telling us where it is, but there seems to be nothing in the spirit dictionary besides the infused pickaxe entry saying that otherworld ores can usually only be mined by otherworld tools(the JEI entry also says to go to your book for more information). Might be good to have an iesnium entry in the book telling you where it is found, how to acquire it, what its uses are, and maybe a bit of lore.

There is also the chance that i am completely blind and it does exist, but i have searched in the book for a minute now and i can find nothing on the subject haha.