I have some old DOS based programs that I bought years ago, and they are no longer supported by their originators. These are medical, data-base type programs that cost a lot of money ($500-2500). I need to change a few things in them (address, telephone number, zip code) that prints on the invoices. I have no idea how to do this as there is no option to change stuff like this. I imagine it can only be changed through some kind of programming language. Can anyone recommend a book that can teach me the right kind of language for something like this?