I believe you know C. If you don't, then learn it. You can write a C program to read and to write to a serial port.
Two good references that I would suggest will be : -
1.) Serial Programming HOWTO by Gary Frerking available in http://www.tldp.org/HOWTO/Serial-Programming-HOWTO/
2.) Serial Programming Guide for POSIX Operating Systems by Michael R. Sweet which can be downloaded from his site which is http://www.easysw.com/~mike/serial/
Both of them discusses the Input/Output concepts of serial devices well. I am sure you will definitely find them interesting.