kindle justification/hyphens

Hi,

Can anyone tell me how to set hyphens in the code, or if there is any other way of eradicating rivers of white spacing between words?

Most of my book looks lovely, but on every few pages there are stretched lines, that distracts me from the content (not good for flow), I would rather see hyphens and have a nice flow, than a neat line, and have to jump from word to word (it's like stopping and starting a car!)

Any help will be much appreciated!

Thanks in advance :)

Zoe