Humble Bundle is very flexible and you can download these e-books as PDF, MOBI, EPUB or PRC formats (depends on publisher) (EPUB seems to be the most widely used Open standard format).
I bought the new DIY one as well as previous, O’Reilly DevOps, O’Reilly Data Science and Maker Skill Builders by Make bundles.
Alternatively you can use a book subscription service like Safari Books https://www.safaribooksonline.com/ $39/mo. It looks like ACM (Association for computing machinery) membership for $99/yr gets you access to Safari books and other publishers. Not to be outdone the IEEE Computer society ($263/yr) offers access to the Skillsoft Library.
If only I had time to read any of the e-books books I have bought from Humble Bundle!