The story of Tommy Taylor (the main character and "real" person in the story of The Unwritten) has been revealed to us in bits and pieces throughout the ongoing story of this graphic novel series. What we do know reveals that he had an awkward upbringing, to say the least, as he was manipulated by his father as part of a marketing ploy to sell books, which was also a political ploy to wrest power away from those who would otherwise abuse it. The story of Tommy Taylor, though (the fictional character in this fictional world, or the character that the "real" Tommy Taylor resembles) has been a bigger mystery, though. We know who he is and what he does (think Harry Potter, but in a much darker world), but we don't know how he became the legend he is. With Tommy Taylor and the Ship That Sank Twice, though, we finally get that story, for both Tommy Taylors.
This isn't a story that could have been written for us any earlier than now. Giving it to us at the beginning would have removed much of the mystery and intrigue that surrounded the start of the story, as it would have given us too many answers. Now, we get to see the stories told side-by-side, and get further reinforcement of what we've seen so far in the series: Stories have power, and the more people read them, the more powerful they become. The theme has become a little familiar, but it doesn't get old.
The Ship That Sank Twice is a stand-alone story set outside the normal flow of The Unwritten, but that by no means makes it a disposable story. It's become a typical marketing thing to have established series tell a new story outside the normal continuity (Werewolves of the Heartland is one of two in the world of Fables), but where those stories are less a part of the standard story, this one is part of the mystery behind the two Tommy Taylors, enough so to be required reading of anyone already into the series. I can see why it was separated (Lizzie and Savoy are not a part of either story), but I still think it should have been worked into the standard series.
If you've hung around with The Unwritten this long, there's no reason why you shouldn't continue with it with this volume. Shoot, even if you don't know the larger story behind the series, the story of the fictional Tommy Taylor's origins might interest you.