The book disappeared once one character uses it, so only one could learn the tenebrium skill.
Unlike Fabulous Five, comes with serious decision-making options, rejecting Mendinus still makes the quest appear.
Say "later" to Brandon, then The Troll's Bounty will be never showed in quest journal.
Even if Brandon has attitude 100, nothing can make him offer his retirement plan again, just because someone said "talk about it later" to him.
IMHO, later =\= never, later =\= reject. This whole crap just don't make sense.
In comparison with "we'll talk about this later", there're much more malicious/injustice/immoral/evil decisions and options in the game.
Only this shtty misleading "talk later" leads to such severe consequences, simply bad design.
GG Larian created this Brandon crap to punish players who may accidently believe your "later" lies, which ruins everything.