If you're a fan of in-depth technical books, Microsoft has just released 150 of them. They cover "serious" topics like Windows 8, SQL, SharePoint and so on, and the total collection runs to tens of thousands of pages. They're all available in PDF format (and others), and all of them are free.
Just point your browser at (here goes):
Note that you may be asked to log in with a Microsoft account when you visit this page (I sometimes get asked, and sometimes don't). If you do, you'll need to sign up for a free account if you don't already have one.