I have just noticed there is a unique lore book in a location which is only accessible during a quest and I have completed the quest. Is there any way to get back in to get the lore book?
"The Grand Sermonizer's Journal" in At-Himah Family Estate
Grand Sermonizer's Journal: Gold Coast Tomes: Q: A Lesson in Silence?; only location inside the at-Himah Family Estate (can access by grouping with someone on the quest, until fixed)
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I am pretty sure that it's impossible to enter areas that are only accessible during certain quests once you've completed that quest. I think you might be out of luck.
You can't get back into quest locations, but the book should be available elsewhere. Try looking near the quest turn-in or the entrance to the location.