Not sure if it's the easiest, but I've gotten the Thay tome and amethyst down to a science. Getting the book is easy enough. And just have something to leave on the pedestal (Indiana Jones like) to keep from the trap smiting you too hard.
As for the stone to unlock it, going in via the broken wall under the forge is the closest location. Fight off the ettercaps and the one spider and get the spider boots off the skeleton for either Astarion or your main if they are a rogue. Split the team and send the other three back to the Blighted Village to wait. Then an invisibility potion and some careful sneaking will keep you from having to deal with the remaining phase spiders. Grab the stone and map back to the Blighted Village to meet up with the rest of the party.
