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?

Libraries

I'm sorry. I couldn't resist.

I actually thought about posting that bundle here. Aside from all of the books available at my local library, they give me access to several digital libraries (Hoopla, Overdrive, Cloud Library), and I can interlibrary loan most other books. Then all state residents can check out books from the university libraries. Being a student or staff at one of those institutions gives additional access to academic ebook collections (Springer, etc.) and journals. Most of the books I actually buy are picked up for $5 or less at my Half Price Books outlet. I've scored some good books at Goodwill stores as well (my stores are often in college towns so you find a lot of textbooks). And I especially like to hit up large scale used book sales, especially on the "bag it" days.

...

NASA has a library of free books.

Project Gutenberg https://www.gutenberg.org/

Internet Archive https://archive.org/details/texts