Python text file into list

so I need some help regarding files and putting them into a list in Python.

I need to read all the txt files in a sub directories into a list word by word.

I was able to do it with just 1 text file

but I don't know how to go about reading the all the files in a sub directory.

I know you have to import os for this, but that is the extent of my knowledge.

This is the code I used to read a file word by word into a single list and it works.

with open('sample.txt', "r") as word_list:
    words = word_list.read().split()

Any help is appreciated. Thanks.