I thoroughly enjoyed reading 'This is social media' by Guy clapperton. It's easy to get into, digest and apply, which means that you can start tinkering about with social media as soon as you start using the book - I say 'using' because it really should be read near a PC. It also acts as a sort of directory, so you can look up the bits you want to return to as and when they become applicable.
Guy takes quite a back to basics approach, carefully explaining the background behind social media and then gradually introducing each area. If you're a social media whizz then this book probably isn't for you, but if you're either completely alien to, or still to use the full social media mix, then it could definitely offer value for your business.
One aspect that really impressed me is that Guy doesn't try to convince his readers that all types of social media are relevant to every business. On the contrary, he encourages you to think about your own aims and objectives before starting with any of them - so you're far less likely to waste your time. There's also a good understanding of the cost of social media to your business, in terms of time, which again makes the book extremely practical.
Overall, thoroughly enjoyed it and hope to start seeing some results soon!