I stumbled on this a little while ago. https://www.packtpub.com/packt/offers/free-learning Each day they make one of their ebooks available for free. You’ll need to create an account, but once you’ve added the book to your account you can then download it in pdf/epub/mobi format at any time in the future.
Each days book is available for 24 hours, and it becomes available shortly after 5pm Pacific time (which is about midnight UTC). So I set an alarm on my phone for 5:30, and visit every day.
I also came across this github repository which maintains links to free programming ebooks (they try to only keep books which are legally free).