Since all the libraries have been closed, we will list legal options to obtain (mostly academic) books for free:

National Emergency Library (USA)

The National Emergency Library makes 1.5 Million Books free to read right now:

Cambridge University E-Books (UK) 

Old Literature made digital by Google Books and the City of Antwerpia (Belgium): 100.000 books printed between 1600 and 1900

Open Library Jstor (USA): Remote access for pupils, students and graduates