Allow books to be copied on the craft grid.

Combine any written book with an empty book to copy it. The
copy is in player hands when using, and the original is put
back on the crafting grid and can be directly copied again.

All ownership and metadata is retained, so the copy of the book
is as writable as the original is, or isn't.

mods/default/craftitems.lua

@@ -88,6 +88,36 @@ minetest.register_craftitem("default:book_written", {
 	on_use = book_on_use,
 })
 
+minetest.register_craft({
+	type = "shapeless",
+	output = "default:book_written",
+	recipe = { "default:book", "default:book_written" }
+})
+
+minetest.register_on_craft(function(itemstack, player, old_craft_grid, craft_inv)
+	if itemstack:get_name() ~= "default:book_written" then
+		return
+	end
+
+	local copy = ItemStack("default:book_written")
+	local original
+	local index
+	for i = 1, player:get_inventory():get_size("craft") do
+		if old_craft_grid[i]:get_name() == "default:book_written" then
+			original = old_craft_grid[i]
+			index = i
+		end
+	end
+	if not original then
+		return
+	end
+	local copymeta = original:get_metadata()
+	-- copy of the book held by player's mouse cursor
+	itemstack:set_metadata(copymeta)
+	-- put the book with metadata back in the craft grid
+	craft_inv:set_stack("craft", index, original)
+end)
+
 minetest.register_craftitem("default:coal_lump", {
 	description = "Coal Lump",
 	inventory_image = "default_coal_lump.png",