Since we're syncing anyway, make the client do a deferred refresh to allow syncing to occur, rather than trying to keep track of prev stacks itself

src/main/java/electroblob/wizardry/client/gui/GuiArcaneWorkbench.java

@@ -150,6 +150,10 @@ public class GuiArcaneWorkbench extends GuiContainer {
 	@Override
 	public void updateScreen(){
 		if(animationTimer > 0) animationTimer--;
+		if(arcaneWorkbenchContainer.needsRefresh){
+			arcaneWorkbenchContainer.refreshBookshelfSlots();
+			arcaneWorkbenchContainer.needsRefresh = false;
+		}
 	}
 
 	@Override

src/main/java/electroblob/wizardry/inventory/ContainerArcaneWorkbench.java

@@ -78,6 +78,8 @@ public class ContainerArcaneWorkbench extends Container implements ISpellSortabl
 	private boolean sortDescending = false;
 	private String searchText = "";
 
+	public boolean needsRefresh;
+
 	public ContainerArcaneWorkbench(IInventory inventory, TileEntityArcaneWorkbench tileentity){
 
 		this.tileentity = tileentity;
@@ -351,6 +353,10 @@ public class ContainerArcaneWorkbench extends Container implements ISpellSortabl
 	 */
 	private boolean mergeStackIntoBookshelves(ItemStack stack){
 
+		// As far as the client is concerned, which stacks were *previously* in which slots never affects whether a
+		// given book can fit or not, so as long as it refreshes it doesn't matter where the book actually gets put
+		if(tileentity.getWorld().isRemote) this.needsRefresh = true; // It's a bit clunky but it works!
+
 		// LinkedHashSet preserves iteration order whilst ignoring duplicates - neat!
 		Set<VirtualSlot> slots = new LinkedHashSet<>(bookshelfSlots.size());