C.S. Lewis’ Screwtape Letters can be found as a free PDF download here:

https://judithwolfe.wp.st-andrews.ac.uk/files/2017/08/Screwtape.pdf

or as a free Audio book (read by John Cleese), here:

https://www.learnoutloud.com/Free-Audio-Video/Literature/European-Classics/The-Screwtape-Letters/3685