It's my understanding that the locations to find Shalidor's books is ramdon. My map marker points me to a location northwest of Solitude. The nearest location I found near that marker is Shadowgreen Cave. I went in but found no book. It that the right location or shoild I be looking for another spot? Thanks in advance!
If it's a random location, we can't very well tell you if you're in the right place. Ask your map or compass - be sure the quest marker is on, and all other quests are turned off. If the quest marker is inside a door on the compass (when you're in the cave), then the item is not in that cave (or section of cave, but I think that one only has one map). On the local map, the arrow-in-a-doorway will tell you which doorway is closet to the target, or will simply have the target marked on the map if you're in the right area.