This was originally a question about finding inexpensive books on a few subjects.
I’m not sure about the language learning books, but for the intercultural communication topic have you tried Coursera? They have some courses on intercultural and interpersonal communication.
There might be some on edX too:
Here’s a free one:
If you want cheap language books, check out your local thrift store or good will. They often have text books and language books. Library sales also rock