Here is a nice video by Martin Fowler on `Introduction to NoSQL`. At the end he talks about polygot databases. It's not like NoSQL is going to replace RDBMs and that RDBMs will vanish for ever. NoSQL and RDBMs have their own space for meeting different requirements and will coexist. Fowler explains it in a very succinct way and it's also fun to watch his talk.
It's interesting to note that he doesn't mention Spanner anywhere. For the impatient to read the Google paper on Spanner, here are some articles on Spanner (1, 2) and a video (1).
I am firm believer that getting a clear understanding of the core concepts is very essential before diving into a new technology. NoSQL Distilled: A Brief Guide to the Emerging World of Polyglot Persistence is a very nice book for those who are getting started with NoSQL. I am also starting a new page with links to NoSQL and will keep it updated as I come across some interesting information.