From the most recent patchnotes:
[quote]How to handle Tenebrium quest book had been increased to unlimited uses[/quote]
The book is still one use only as it was before. It disappears after read just like it used to. It doesn't have unlimited uses.
Edit: I started a new game and the book works for unlimited uses now. It's ridiculous though, I shouldn't have to start a new game for something as simple as this.
Last edited by nbog; 16/09/14 10:03 AM.