I think that's the name of the spells. I can't find the books for them anywhere and I've completely cleared Solstheim as of earlier today.

I thought I would be able to buy it from the apprentice at Tel Mithryn (the UESP says so too) but I've done all of the quests there and Neloth's apprentice still doesn't have it (but he does have the Conjure Ash Guardian spell aswell as Poison and Frenzy Rune).

I've been days going back and checking (also waited more than a few times) but the books are just not being added to his inventory.

So is this a glitch or do I find the spell elsewhere/activate them in his inventory by other means? Any help is much appreciated!

Do you need a high enough alteration level?

I got my Alteration skill to 50 today and the Ash Shell spell appeared so it seems that you do. Thanks!

Also, there is a glitch with his inventory. You have to save, kill him, then re-load the save so his inventory changes. lol