I don’t know if this fits this particular category exactly, but there is a Humble Bundle of books on programming languages, and one on Julia (The O’Reilly “Think Julia”) is in the $1 section.
Humble Bundle is a very charitable endeavour. If anyone wants the PDF of Think Julia, do not hesitate to support them!
However, I’d like to point out that Think Julia is also available for free as a web version:
Thanks! I did not know this!