I, for one, hadn't realized they'd made the books free to download (though I had intended to check that). I think most people assumed articles and journals when they read about the free DL access.

That's awesome. I certainly missed this bulletin!

This is great. Do you know other similar resources that have been made available during this time?

I don't, but keeping an eye out here is probably the best way to find out; I'm pretty sure that's how I saw the announcement above.

Lots of publishers are making their textbooks temporarily available through resources like VitalSource and Redshelf, but (a) VitalSource at least is still reading DRMd material through a clumsy app (I haven't tried Redshelf); and (b) they seem more geared towards college textbooks than to scholarly works.

Cambridge allowed everyone to access their library but then they changed it to "available online to students through their university library regardless of whether they were previously purchased" https://www.cambridge.org/about-us/covid-19/