I’ve shared a couple of these on Twitter, but putting them in the same place. Will add others if I see them, or please add as comments.
Free Quarantine Ebooks from Verso – Reading in a time of coronavirus: download your free ebooks until April 2.
Ten Free Ebooks from Haymarket Books: Angela Y Davis, Naomi Klein and others
“You can search all open access content on JSTOR without a login – there’s more than 6,000 ebooks and over 150 journals”: [Clarification: this appears just to be the stuff that was already open access, not a wider opening up of the archive] jstor.org/open/
And the Cambridge UP textbooks previously mentioned are available here.
Update: World e-book here (looks good for people with children)
JHU Press – free access to Project Muse: “The collection’s 1,400 books and 97 journals will be accessible for free through May 31 to help university students complete coursework at home”