Everybody is always asking "is there a ferry to...?" And the answer is usually the same: if you can't stand on the dock itself and inquire, do the next best thing and read this unique, comprehensive book.
The book gives the ferry schedules for every island and for various mainland ports. You also get an overview of the various route patterns. This is especially valuable given the somewhat random nature of the schedule of Greek ferries. The route patterns and timings are key for planning a general strategy to serious island hopping.
Beyond this basic information, the annual guide offers wonderful gossipy reviews of individual boats, companies, and the changing picture of ferry travel from year to year. There are also little intros to the various islands. These island reviews are notable for detailed maps of the port cities showing where the boats land. Note: many islands have more than one location where boats dock. You could wind a hundred yards or more away from where you need to be. With this guide you'll not be surprised.
But note that reference to maps of the island port cities. This is near-unique feature as you generally can't find that level of detail for all the islands in one place. The maps include info on car hire, some hotels, places to eat. There are also often general island maps focusing on bus travel and beaches.
Even ferry schedules published on site in Greece are notoriously "selective" in their accuracy, so how good can this book be? Obviously, I've not checked even half of the contents, but for the few journeys I and my friends have taken, it has proved accurate.
So I would absolutely rely on this for planning a trip, but in no way rely on it day to day when I was in Greece. Go to the ferry agents or perhaps to the dock itself. Double check WITH THE SPECIFIC COMPANY YOU ARE USING, because what company A tells you about Company B is next to useless. Bear in mind that schedules do reduce in the off season.
Bottom line: this book is essential. Rough Guides call it "superb, user-friendly." Check out the book's web site also.