See * for disclaimer
Under the Sparkling Sea (Illustrated Novel, April 23rd 2013)
I wasn't going to do this one, as I was limiting myself to the show, the comics, and a few of the books that turned out to be referenced in the show itself. However, I discovered recently that this book was referenced in the comics, so it's valid. Though I'm going through it now, I'll put it into the jumplinks on the side there as being in the publish date.
It also has taken me awhile to get this one, due to a weird glitch in the Kindle copy of the storybook. On the Windows Kindle reader, the book auto-minimizes. Every single time you open it. Very weird. However, I discovered by accident that there is a Cloud Kindle reader that works just fine.
To start with the art style is a bit distracting. It's somewhere between G3 and G4 and looks very odd. However, it does seem to suit the style of storytelling. Despite the publish date, we're dealing with pre-Princess Twilight here, so this takes place before the end of Season 3.
We have a reference to an 'Annual Aquastria Race' taking place at First Moon Tide, hosted by King Leo of Aquastria. Twilight and Spike are both aware of this Aquastria right at the beginning, and that it's an underwater kingdom. First Moon Tide is a weird way of marking a specific date, but perhaps we're dealing so far underwater that the tide is detectable by changes in currents, but the moon itself isn't immediately visible. I'm going to have to assume it's the first full moon of the year. Which is an interesting question, when does Equestria put the New Year? In the west we've become used to it being in the winter, but other cultures usually have it being the birth of the new year (start of spring), or the death of the old year (end of fall). The early Romans had the winter actually being not part of the year at all.
Twilight has the relevant spell on hand, as usual. Giving the Mane 6 gills and fins, but not transforming them into full seaponies. Fluttershy reveals that the ponies consider the seaponies to be their 'cousins'. Twilight's spell doesn't work on non-ponies, so spike gets a fishbowl stuck on his head.
A new creature for the bestiary: A manta-hawk. Weird blending of manta ray and raptor, obviously used for heavy transport underwater.
First visual of the new G4 seaponies (allowing for the unusual art style of course.) They're not far off the seaponies of G1, though a bit more RL seahorsie. They're approximately the same size as regular ponies, if perhaps a bit smaller. No obvious cutie-marks.
Architecture is typical fantasy underwater, built to resemble a variety of seashells and corals.
And here we have King Leo himself, who's appearance is far more lionfish. In fact there are other creatures in the background who are also far more fish than seahorse in design, having tailfins. Are those all 'seaponies' or are seaponies just a segment of the population?
We are also introduced to jellyflies, which appear to be a combination of butterfly, firefly, and jellyfish.
And here we have the mermares. Which appear to be giant Cornetfishes. Basically they're to the seaponies what the Saddle Arabians are to regular ponies. Larger and formed slightly different. In this case they have manes that go all the way down their back, and tailfins. Mermares are either shy, or xenophobic. I'd say shy, as xenophobes wouldn't even show up to mingle like this.
Apparently turtles are part of this society as well.
However, there are a mix of creatures that live on the outskirts of Aquastria. Narwhales appear sentient, while water weasels, foxfins, crabbits, and star mice appear to be animal-level creatures.
The race is a standard relay race, and seems to change location yearly as this is the first time through Crabbit Canyon.
Nothing else really occurs world-building wise, but there was quite a lot for a simple storybook.