HOW DO PACKETS WORK D:

ClientProxy.java

package net.aerobit.expbooks.client;

import net.aerobit.expbooks.CommonProxy;
import net.minecraftforge.client.MinecraftForgeClient;

public class ClientProxy extends CommonProxy {
	@Override
	public void registerRenderers() {
		MinecraftForgeClient.preloadTexture(ITEMS_PNG);
		MinecraftForgeClient.preloadTexture(BLOCK_PNG);
	}

}

CommonProxy.java

package net.aerobit.expbooks;

import net.aerobit.expbooks.client.GuiExpBook;
import net.minecraft.src.EntityPlayer;
import net.minecraft.src.World;
import cpw.mods.fml.common.network.IGuiHandler;

public class CommonProxy implements IGuiHandler {
	public static String ITEMS_PNG = "/aerobit/expbooks/items.png";
	public static String BLOCK_PNG = "/aerobit/expbooks/block.png";
	
	// Client stuff
	public void registerRenderers() {
		// Nothing here as this is the server side proxy
	}

    @Override
    public Object getServerGuiElement(int id, EntityPlayer player, World world, int x, int y, int z) {
    	return null;
    }

    //returns an instance of the Gui I made earlier
    @Override
    public Object getClientGuiElement(int id, EntityPlayer player, World world, int x, int y, int z) {
    	return new GuiExpBook(player);
    }
}

ExpBooks.java

package net.aerobit.expbooks;

import net.minecraft.src.Block;
import net.minecraft.src.Item;
import net.minecraft.src.ItemStack;
import net.minecraft.src.Packet;
import cpw.mods.fml.common.FMLCommonHandler;
import cpw.mods.fml.common.Mod;
import cpw.mods.fml.common.Mod.Init;
import cpw.mods.fml.common.Mod.Instance;
import cpw.mods.fml.common.Mod.PostInit;
import cpw.mods.fml.common.Mod.PreInit;
import cpw.mods.fml.common.Side;
import cpw.mods.fml.common.SidedProxy;
import cpw.mods.fml.common.event.FMLInitializationEvent;
import cpw.mods.fml.common.event.FMLPostInitializationEvent;
import cpw.mods.fml.common.event.FMLPreInitializationEvent;
import cpw.mods.fml.common.network.NetworkMod;
import cpw.mods.fml.common.network.NetworkRegistry;
import cpw.mods.fml.common.network.PacketDispatcher;
import cpw.mods.fml.common.registry.GameRegistry;
import cpw.mods.fml.common.registry.LanguageRegistry;

@Mod(modid="ExpBooks", name="ExpBooks", version="0.0.1")
@NetworkMod(clientSideRequired=true, serverSideRequired=false, 
channels={"ExpBooks"}, packetHandler = PacketHandler.class)
public class ExpBooks {

        // The instance of your mod that Forge uses.
	@Instance("ExpBooks")
	public static ExpBooks instance;
	
	// Says where the client and server 'proxy' code is loaded.
	@SidedProxy(clientSide="net.aerobit.expbooks.client.ClientProxy", serverSide="net.aerobit.expbooks.client.CommonProxy")
	public static CommonProxy proxy;
	
	private final static Item expBookItem = new ItemExpBook(5000);
	@PreInit
	public void preInit(FMLPreInitializationEvent event) {
		// Stub Method
	}
	
	@Init
	public void load(FMLInitializationEvent event) {
		proxy.registerRenderers();
		GameRegistry.addRecipe(new ItemStack(expBookItem), "x", 
		        'x', new ItemStack(Block.dirt));
		LanguageRegistry.addName(expBookItem, "Exp Book");
        NetworkRegistry.instance().registerGuiHandler(this, proxy);
	}
	
	@PostInit
	public void postInit(FMLPostInitializationEvent event) {
		// Stub Method
	}
	public static void sendCustomPacket(String type, byte[] data) {
		Packet p = PacketDispatcher.getPacket(type, data);
		if (FMLCommonHandler.instance().getSide().equals(Side.CLIENT)) {
			PacketDispatcher.sendPacketToServer(p);
		}
	}
}

GuiExpBook.java

package net.aerobit.expbooks.client;

import net.aerobit.expbooks.ExpBooks;
import net.aerobit.expbooks.ItemExpBook;
import net.minecraft.src.EntityPlayer;
import net.minecraft.src.GuiButton;
import net.minecraft.src.GuiScreen;

import org.lwjgl.opengl.GL11;

public class GuiExpBook extends GuiScreen {
	public final int xSizeOfTexture = 176;
	public final int ySizeOfTexture = 88;
	private int posX = (this.width - xSizeOfTexture) / 2;
	private int posY = (this.height - ySizeOfTexture) / 2;
	private GuiButton plus;
	private GuiButton minus;
	
	public GuiExpBook(EntityPlayer entityPlayer) {
		
	}
	
	@Override
	public void updateScreen() {
		plus.enabled = mc.thePlayer.inventory.getCurrentItem().getItemDamage() > 0;
		minus.enabled = mc.thePlayer.inventory.getCurrentItem().getItemDamage() < mc.thePlayer.inventory.getCurrentItem().getMaxDamage();
	}
	
	@Override
	public void drawScreen(int x, int y, float f) {
		drawDefaultBackground();
		int var4 = this.mc.renderEngine.getTexture("/aerobit/expbooks/GuiExpBook.png");
		GL11.glColor4f(1.0F, 1.0F, 1.0F, 1.0F);
		this.mc.renderEngine.bindTexture(var4);
		int posX = (this.width - xSizeOfTexture) / 2;
		int posY = (this.height - ySizeOfTexture) / 2;
		drawTexturedModalRect(posX, posY, 0, 0, xSizeOfTexture, ySizeOfTexture);
		drawString(fontRenderer, "Level: " + (30-mc.thePlayer.inventory.getCurrentItem().getItemDamage()), posX+64, posY+20, 0xEEEEEE);
		super.drawScreen(x, y, f);
	}
	@Override
	public void initGui() {
		this.controlList.clear();
		posX = (this.width - xSizeOfTexture) / 2;
		posY = (this.height - ySizeOfTexture) / 2;
		this.controlList.add(plus = new GuiButton(0, posX + 20, posY + ySizeOfTexture / 2, 20, 20, "+"));
		this.controlList.add(minus = new GuiButton(1, posX + xSizeOfTexture-40, posY + ySizeOfTexture / 2, 20, 20, "-"));
		plus.enabled = mc.thePlayer.inventory.getCurrentItem().getItemDamage() > 0;
		minus.enabled = mc.thePlayer.inventory.getCurrentItem().getItemDamage() < mc.thePlayer.inventory.getCurrentItem().getMaxDamage();
	}
	@Override
	public void actionPerformed(GuiButton button) {
		switch(button.id) {
			case 0:
				ExpBooks.sendCustomPacket("ExpBooks", new byte[]{ (byte) 2} );
				break;
			case 1:
				ExpBooks.sendCustomPacket("ExpBooks", new byte[]{ (byte) 1} );
				break;
			default:
				break;
		}
	}
	
	@Override
	protected void keyTyped(char par1, int par2) {
		if (par2 == 1 || par2 == this.mc.gameSettings.keyBindInventory.keyCode) {
			this.mc.thePlayer.closeScreen();
		}
	}
	@Override
	public boolean doesGuiPauseGame()
    {
        return false;
    }
}

ItemExpBook.java

package net.aerobit.expbooks;

import net.minecraft.src.CreativeTabs;
import net.minecraft.src.EntityPlayer;
import net.minecraft.src.Item;
import net.minecraft.src.ItemStack;
import net.minecraft.src.NBTTagCompound;
import net.minecraft.src.NBTTagList;
import net.minecraft.src.NBTTagString;
import net.minecraft.src.World;

public class ItemExpBook extends Item {
	public ItemExpBook(int i) {
		super(i);
		this.setMaxStackSize(1);
		this.setCreativeTab(CreativeTabs.tabMisc);
		this.setIconIndex(0);
		this.setItemName("expBookItem");
        this.setMaxDamage(30);
	}
	
	public String getTextureFile () {
		return CommonProxy.ITEMS_PNG;
	}
	
	public static void setDescription(String s, ItemStack itemStack) {
        if (itemStack.stackTagCompound == null) {
        	itemStack.setTagCompound(new NBTTagCompound());
        }
        if (!itemStack.stackTagCompound.hasKey("display")) {
        	itemStack.stackTagCompound.setTag("display", new NBTTagCompound());
        }
        NBTTagCompound tagCompound = itemStack.stackTagCompound.getCompoundTag("display");
        if (!tagCompound.hasKey("Lore")) {
        	tagCompound.setTag("Lore", new NBTTagList());
        }
        NBTTagList loreList = tagCompound.getTagList("Lore");
        
        if(loreList.tagCount() > 0) {
        	for(int i = 0; i <= loreList.tagCount(); i++) {
        		loreList.removeTag(0);
        	}
        }
        loreList.appendTag(new NBTTagString("Lore",s));
    }
	public String getDescription(ItemStack itemStack) {
		return "";
	}
	@Override
    public ItemStack onItemRightClick(ItemStack itemStack, World world, EntityPlayer entityPlayer) {
		entityPlayer.openGui(ExpBooks.instance, 1, world, (int) entityPlayer.posX, (int) entityPlayer.posY, (int) entityPlayer.posZ);
    	return itemStack;
    }
	@Override
    public void onCreated(ItemStack itemStack, World world, EntityPlayer entityPlayer) {
    	itemStack.setItemDamage(itemStack.getMaxDamage());
    }
}

PacketHandler.java

package net.aerobit.expbooks;

import java.io.ByteArrayInputStream;
import java.io.DataInputStream;
import java.io.IOException;

import net.minecraft.client.Minecraft;
import net.minecraft.src.INetworkManager;
import net.minecraft.src.Packet250CustomPayload;
import cpw.mods.fml.common.network.IPacketHandler;
import cpw.mods.fml.common.network.Player;

public class PacketHandler implements IPacketHandler {
	@Override
	public void onPacketData(INetworkManager manager, Packet250CustomPayload packet, Player playerEntity) {
		if (packet.channel.equals("ExpBooks")) {
			handleBook(packet);
		}
	}
	private void handleBook(Packet250CustomPayload packet) {
		Minecraft mc = Minecraft.getMinecraft();
		DataInputStream inputStream = new DataInputStream(new ByteArrayInputStream(packet.data));
		
		byte input;
		try {
			input = inputStream.readByte();
		} catch (IOException e) {
			e.printStackTrace();
			return;
		}
		System.out.println(input);
		if(input == (byte) 1) {
			if(mc.thePlayer.inventory.getCurrentItem().getItemDamage() < 30) {
				mc.thePlayer.inventory.getCurrentItem().damageItem(+1,mc.thePlayer);  //this is the code to make it take one off of the stack
				mc.thePlayer.experienceLevel += 1;
				ItemExpBook.setDescription("Level: " + (30-mc.thePlayer.inventory.getCurrentItem().getItemDamage()),mc.thePlayer.inventory.getCurrentItem());
			}
		}
		if(input == (byte) 2) {
			if(mc.thePlayer.experienceLevel > 0 && mc.thePlayer.inventory.getCurrentItem().getItemDamage() > 0) {
				mc.thePlayer.inventory.getCurrentItem().damageItem(-1,mc.thePlayer);  //this is the code to make it take one off of the stack
				mc.thePlayer.experienceLevel -= 1;
				ItemExpBook.setDescription("Level: " + (30-mc.thePlayer.inventory.getCurrentItem().getItemDamage()),mc.thePlayer.inventory.getCurrentItem());
			}
		}
	}
	public static String convertStreamToString(java.io.InputStream is) {
	    java.util.Scanner s = new java.util.Scanner(is, "UTF-16").useDelimiter("\\A");
	    return s.hasNext() ? s.next() : "";
	}
}