Professor Walthan's Guide to Flora and Fauna, Vol 6
There is an entire series of books with this name, for the list of them see Professor Walthan's Guide to Flora and Fauna.
This book can be obtained by speaking to Professor Walthan with a Ghastroot in the inventory.
Volume 6: Ghastroot
Contents
I've still not been able to find any "ghastroot" naturally occuring in the Valley. From time to time I've heard strange whispers of a special tea with that name being enjoyed, but not once I have been able to actually find the damn root. I've interviewed at least two tea experts, though I'm not sure how much of an "expert" I would consider my coworker, Sumona. She just seems to think Ghastroot is a magical tea root that grows underwater. My other expert though... he offered me something far more interesting. A conspiracy! An adventure! He believes that there is a secret sect of mages and alchemists hiding outside of time, to protect the power of the root - and its tea - from outsiders! Can you believe it? A PLANT CONSPIRACY! I must learn more.