Scientific publisher Springer has released a large number of titles from their backlist in response to the international COVID-19 crisis. Computer scientist and UK startup co-founder Harish Narayanan has helpfully provided a list by subject here:
https://hnarayanan.github.io/springer-books/
Drill down into the subject, and click on the links in it to get the books. (You have to solve a Captcha to prove you aren't a bot, but that's a one time operation. No other action is needed and you will not be asked for identifying information.)
Most titles seem to be available in ePub and PDF formats, but a few are PDF only.
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Dennis
Who has found an assortment of titles on things he is interested in, and is grateful these are eBooks that don't take up shelf space.