We’re always happy to find a large collection of free educational books, and it looks like Springer has recently made available over 50,000 books covering STEM subjects.
The academic and research books are available as PDF downloads. You can search and filter the list by discipline and topics. Hope you have enough room on your hard drive!
Try the link below, which searches all books written in English, except the preview-only ones. There’s also this list from GitHub.