Every month the Humble Bundle kicks out a promotion for "pay what you want" for books, games, etc.
This month is all about unix/linux.https://www.humblebundle.com/books/unix-book-bundle?charity=178993
The money you pay is donated to charities. I took the liberty of reformatting the link to pre-select the Air Force Association in appreciation for everything they have done for us.
It is definitely worth $1 for the first 5 books, and I'll be personally paying the $15 to get all 16 books.