Further on this topic of this being a themed book - I really hope so, as the themed books from 3E were my favourite. Iwas waiting for more of them when the end came.
I don't mean Warrior themed with 2-3 things in there to make it 'useful' for wizard PCs. I thought 4E did that better with their power source books.
The 3E themed books I really liked were those about groups of monsters or terrain types. So, what I am saying is I hope this is 5E's 'Stormwrack'. That and 'Sandstorm' are books I keep going back to when running nautical or desert styled campaigns. Of course there is room for improvement, such as an adventure, mini-setting, etc, but those are the type of books I would really LOVE to see in the 5E game.
Some 3rd party publishers have started down this route with fey and nautical themed books already. I have neither, unfortunately, but if I run such a themed game, I will be getting them. (Oh, one member of our group did back the nautical/pirate Kickstarter).