Packets in a multiplayer game

Hello everyone, I want to make simple a multiplayer game in c++ and I want a good book to learn about that.For example a MMORPG game, what to be send between client and server? How a client know if a monster or a npc, or a player is near him? for example if a monster move and it's near player, client will ask server if a monsters is near and server will send back the monster coordonates? Can someone explain me how this works?

Thanks

I think for this you need some Knowledge regarding Networking. Here i can recommend your Cisco Academy for Infos.

Well, there are many possibilities how you design this.
You can make regions, and you can update it based on each region in the game.
This is just an example, you should google this, there is a lot of documentation and ideas to find over there :)