I have heard about the book of Judith. Does this book exist and if so who wrote it and where can I find a copy in English?
Yes, the book of Judith exists. It was included in the first Bible "canon" that was set by any Church council (Carthage, 397), and remained in the books included with the Old Testament by the first millennium Church. It is part of a set of books called alternatively "deuterocanonical" and "apocrypha" - though the latter term is somewhat misleading, as no Greek Christians ever referred to the books by that term (which means something like "hidden" or "secret" in Greek). (An overview of the formalization of the Bible canon can be found here.)
You can find several translations online, including this one, which contains the King James version.
The author of Judith is not known and it is not known for sure whether the book was written originally in Greek or Hebrew. Although the story in the book takes place during the time of Nebuchadnezzar, it was probably written between 135 and 105 BC.*
* Introduction in The Orthodox Study Bible