Continually updated lists of free publisher material:
Here are just a few major academic publishers with free offerings right now:
EBSCO & Harvard Business Publishing eBooks
Internet Archive's National Emergency Library - "a collection of books that supports emergency remote teaching, research activities, independent scholarship, and intellectual stimulation while universities, schools, training centers, and libraries are closed"
Search the Internet Archive's National Emergency Library
Macmillan - "As more schools close due to the coronavirus, we would like to offer you and your students FREE access to LaunchPad, Sapling, iClicker, or FlipIt to get you through the rest of the semester or term if you were not already using one of these digital products"
Oxford University Press - "We have partnered with our eBook providers RedShelf and VitalSource to supply FREE access to all OUP Higher Education titles on their sites for the balance of the Spring semester." (with some restrictions)
Pearson via VitalSource
Pearson via RedShelf
University of California Press Journals
Wiley - "Wiley Offers Free Access to Support Educators, Researchers & Professionals Amid Growing COVID-19 Impact"