Every year Microsoft organises a whole lot of technical ebooks that can be downloaded for free.
This years can be found here
Note that these are not MS owned books and are provided by their authors/owners so availability may be removed or limited.
These either can be downloaded individually or there is a script provided to use in PowerShell to get them all (about 3.5Gb).
If you need help with PowerShell let me know and I can post a how to.