Same happened to my group except one person in it finished the quest, i lost 3 gold over this and im a little flustered too

This is known, as I've been told before its not a bug (as I reported it the 1st time we had this quest), you only have a chance to get a tome, its not guaranteed your get it.
we had to repeat boss fight 3 times for all 5 of us to get the tome.

one tip, you only have to kill one mob per section not all of them, before you can move on, but i'm not 100% sure if it affect any extra mobs in boss fight. but means you can get to boss a lot quicker.

I do think we need more than 24 hrs to get all 3 done, esp as you need to repeat multiple times, plus arrange for a group of people to go.

The 2nd boss is very hard also we had a very good team (44-46 all with blue mounts) , and just killed him, but we have to use every summon we had, inc idols etc, and most of us died.

Just to add to what Bones already said - not everyone receives the quest item drops from this part of event in your group and it is intended that you have to do the bosses few times for everyone to get the quest drop in your group.

Cant help but wish the admin would go through this and rethink this event it starts great but the last 3 parts are horrible