I read somewhere that there was a novel that accompanied the movie. It gave insight into how Ruth's husband died, and how they (the dewitt-bukater's) lost money, and how Ruth herself was "New Money" Am I crazy, or did this actually come out into print?
I don't recall any books of that sort coming out. There's a companion book called "James Cameron's Titanic" and a book that features the script and the changes they made to it as they filmed, but I've never seen a "novelization" per se.
The "James Cameron's Titanic" was basically a behind-the-scenes book that talked about the dives to the wreck and building the sets, etc. The script book might be your best bet as I seem to remember the script having a few more things said about Cal's family and the history behind the characters, but it really didn't amount to anything.