Python is open source. You can download the source for it, examine it, modify it, compile it and play with to whatever degree you please. The articles mentioned though would be the starting point. Also there are books on writing compilers and interpreters. "Writing Compilers and Intrepreters" by Ronald Mak would be one. So if you really want to write a scripting language I would suggest starting with the articles mentioned, work through a book like the one above and then work through the source for something like Python. If you keep at it and have the time then about a year from now you will pretty much be an expert at it.
It is a lot of work, but I think it is well worth it. Especially if you are starting out. The knowledge gives you the ability to automate your job to an incredible degree. Generally I find the parsing to be more useful than actual scripting. One example was one change potentially impacted 600 programs, but parsing the programs let me cut it to only 20 that where actually changing. Another was a need to write 1200 scripts to execute a job stream for a conversion. Generating the scripts only took two weeks for task that was alotted three man months. So a little bit of knowledge can let you seemingly do the impossible.