Truly understanding networking?
W. Richard Stevens' books 'UNIX Network Programming' and 'TCP/IP Illustrated' are must reads, no matter which career you go with.
If you would like more detail on the underlying technologies and theory, 'Computer Networks', by Andrew Tanenbaum was the seminal text when I was studying; again, highly recommended.
Exploring the material covered in ICND1, ICND2, which is what prepares you for CCNA would be a good learning path to take.