If you're looking for good reference and study books, take a look at Humble Bundle. I've used them for years and thought it worth mentioning here in case anyone else might be interested. For example someone starting out with Linux and looking for some good reference books to get stuck into.
Humble Bundle β Books
They do bundles of books covering Linux, Windows, sysadmin, cloud, scripting, programming, project management, security, Docker and so on. And you can choose how much to pay and support selected charities too. The books are DRM-free and fully downloadable as PDFs and EPUB files.
At the moment they've got a 21 book Linux bundle, a 15 book Cloud Infrastructure O'Reilly bundle, and 19 book programming bundle. These bundles by different publishers come and go, with different books each time, so always good to see what comes up.