How to Handle TenebriumThe book can be read by many heroes and will not disappear. But, when I read it after I had completed the quest 'The Troll's Bounty', it had no effect. The Tenebrium skill was not increased by 1 for any of my heroes. I have not tested what happens if I read the book first and then complete the 'The Troll's Bounty' quest, but I believe it might work then.
You are correct (although the book only has unlimited uses if you started a new game after the patch). I ran into the same problem and reloaded an earlier save. I read the book before I turned in The Troll's Bounty quest and I was able to have a +2 Tenebrium bonus on all my characters in my current party.
Immaculate Summoner in The White Witch Cabin
If you do not convince the Immaculate Summoner guarding the cabin of the White Witch to leave, he will summon first 4 then 5 shadow demons that you have to fight. When you get the barrier down and try to enter the cabin, you have to fight the Immaculate Summoner himself. At least that was what happened earlier. In this version the Immaculate Summoner is stuck inside the cabin and I could kill him without any fight.
I had the same problem with the summoner after I started a new game for the new companions patch.