There are lots of tools for a writer to choose from, whether it's (now anyway) the internet, others, classes, workshops, etc or books. Generally, nowadays, people will be more inclined to go on tumblr or deviantart or whatever websites that will offer resources and such to aspiring or veteran writers.

But there is still a good list of writing books that can become useful for one or many writers (while others prefer to improve by themselves).

I have a few I always keep near my desk, just because I find them useful or amusing.

• Emotion Thesaurus (of which I speak more in lenght here)
  

Unfortunately, all my other writing books are in french, though if any are interested I can list them anyway but yeah xD

How about you? Do you have books you rely on, or sometimes read, or find interesting that can help a writer improve? Whether their dialogues, characters, settings, plots, etc?