The answer it's the same Ethernet library you are already using (there's no specific "EthernetTcp" library, just Ethernet).
The main difference is that TCP requires the establishment of a connection, so there are two different behaviors: the client requests the connection, while the server waits for incoming connection requests. From your code I think that the Arduino will work in the role of the server (EthernetServer). It will wait until some data (a request from the outside) is available(), then a Client will be created on the spot to read the incoming message, and finally the server will send the response to the outside client and close the connection.