Computer Networks - A Top Down Approach by Kurose Ross gives a very good overview of computer networks and dives deep into each of the layers of the networking stack.
RFC-1180 TCP/IP Tutorial, very good and clear introduction to TCP/IP
Genuine related question - what is learning TCP/IP good for? When is it useful to know?
1. When you need to communicate across the network and don't have a library to do it for you.
2. Troubleshooting
3. Insights. Knowing how things work can sometimes give you big insights into potential solutions or pitfalls to avoid
Additionally, tuning/optimization when dealing with high-latency networks, possibly with more frequent packet loss than you'd get in a local data center (think data replication from Hong-Kong to London in a global bank, etc).