* Hadoop: the definitive guide
* Cassandra: The definitive guide
And they are both excellent books. I have heard good things about:
* Think Stats/Think Bayes
* Learning Spark/High Performance Spark
EDIT: I just bought the bundle. The PDFs seem legit and DRM-free -- O'Reilly no longer offers DRM-free ebooks, so this is useful.
If you sign up for an account you can also save your purchase to your account and then access all the purchases made through the library.