Text Files
You can open a text file and dump it's contents into a string like so:
textFile = open(filename, 'r')
#open() returns a file object
#'r' opens the file for reading. 'w' would be writing
textString = textFile.read()
#This takes the file object opened with the open() and turns it into a string which
#you can now use textString in a text widget.
More info on text files and Python
Linking listbox with text files
To link listbox items with a text files, I suppose you could have all the things you are putting in the listbox in a dictionary, more info here. Unlike arrays or lists, which are indexed by a range of numbers, dictionaries are indexed by keys, which can be any immutable type; strings and numbers can always be keys. So for instance you could have a filename as the key and whatever you want in the listbox as the value.
I hope I helped a bit.