For The Black Book - 1858 (and many similar quests). When you are grouped, Formius drops the Black Book for only 1 character (whoever loots it). Formius is on a standard respawn timer, which means other group members have to wait. Then, when Formius respawns, he may not drop a book at all, meaning further waits. There are other quests like this (Sharptooth and Brannoq in Lomshir Plain near Dark Horse downs, for example). The mobs should drop their quest item for each member of the party, and not just one for each kill.