Is there any way to check if a certain content such as text would be unique on the internet ? I have a book I was thinking of writing from, written by my dad and I want to make a website about it but want to make sure no one's copied the text word by word from the book before.
Not really. You can use things like copyscape to try and check it but it cant really be that accurate because you never know what else people have added to the page, navigation, comments etc. So something that is duplicate may be considered unique... copyscape is good to check if the search engines would consider it duplicate content but a real person reading it would be able to tell instantly if it is copied from something they already read somewhere else.
You can use Copyscape to test the contents out but as dddougal has said, some articles can be rewritten in another way so if it is really your dad's writings, just post it up and your readers will tell you.
I like CopyScape also you can just copy few lines of your content in "" and then make a search on Google also.
Really you should be looking for ways to protect your unique content, or are you checking content you did not write?