I agree with Vel Nias' answer that Shreve's books are good. I would like to add that if you have literally no idea at all in finance, as I did when I first started reading about financial mathematics, it would be a very good idea to read a non-mathematics book about finance to learn what stocks are etc. Most authors of economics books try hard to show how clever they are, so their books can be quite difficult to read. On the other hand, children's books can sometimes be good, such as Growing Money by Karlitz and Honig. I also recommend The Great Game by John Steele Gordon, which is a relatively down-to-earth history of Wall Street.
(By the way, it's almost impossible to find a book about finance which is unbiased in the sense that it doesn't try to offer some kind of investment advice.)