Just want to add this note in case anyone is confused here like I was. To purchase the spell from Warlowe Rida in Plane of Tranq, you first have to talk to her and follow all her dialogs.
When I went in the zone, she had the spell in her inventory, but when I had the merchant window set to "Show items I can use" the spell did not show up. I talked to her, went thru all her dialogs and then it showed up as an item I could use.
Not sure if I had bought it before speaking, if I would have had trouble putting it in my spell book.