Humble Bundle periodically sells collections of e-books for $15 or less. Two current book bundles are
Essential Knowledge series from MIT Press (expires in 13 days, as of 2018-08-21) and Big Data by Packt (expires in 6 days).
"The MIT Press Essential Knowledge series offers accessible, concise, beautifully produced books on topics of current interest. Written by leading thinkers, the books in this series deliver expert overviews of subjects that range from the cultural and the historical to the scientific and the technical."
What are some other sources of inexpensive books?
