He is located at +96, +271 in the corridor just past Tarlyn Caelith (Missions). He is level 90, and cons indifferent. When a player who is working on the quest enters the adjacent room, he goes active and makes his way to where that player is and attacks.
It apparently takes a few seconds for him to go active and it appears that, if you are able to run through the room and into the corridor where quest giver Varesa is standing, you can go to the bottom of the stairway next to her to the dead end of the corridor and set up your group.
He will aggro each player that has the quest and who is in need of Volume 588.
After the group is set, have each play who needs the book (one at a time), invis and step out of the corridor into the main room. You will get an emote from him and he will start moving toward you. Return to your group, have him targeted, and have your tank (player, mercenary, or mage pet(s)) attack him mid-stairway. Once he is down, loot the book.
NOTE: If you are one who likes to turn in your items as the quest progresses, DO NOT do so for this book. If you still have the quest but no longer have Volume 588, he will go active again. So keep Volume 588 in your possession until the end of the quest.