This Sheikah symbol is decorating Nintendo's E3 website for now. We've seen this symbol before on the book that Link was carrying in the short footage from last November's Nintendo Direct and in the new artwork from last week:
This book already caught many people's eyes in November and some even thought back then that the book will be important and could be in the title of the game. And right now it looks like they were right. In the past I speculated that the game might be called something like "Bow of the Goddesses", since the Bow seems to be important, but by now we can assume that the book might be the most important piece in the game and that it's likely related to the Sheikah in some way.
It could be a Spell Book. When I personally think about a huge Zelda game world, Zelda II - The Adventure of Link always comes to mind. This game still has the largest overworld in the Zelda series, but it also had a different way to use magic in the form of spells (instead of items), which may come back in this game. I suppose, you will be able to browse the book with the Wii U GamePad (similar to the Stamp Book in Spirit Tracks) and maybe even activate the spells by drawing symbols. We've already seen that the game has a Magic Meter (or at least a green meter) during the Game Awards 2014 presentation, so it's not too far fetched and could lead to some interesting things.
In any case, I like the mysteriousness of it all. This is great teasing! The Sheikah always have been the most mysterious tribe and people love their mysteries. So, this is ingenious marketing for Zelda fans. Nintendo just has to post a Sheikah symbol and every Zelda fan out there will be hyped. But it's the mystery itself, which makes the Sheikah so interesting and popular. If you take that away, it might not be that anymore. It's the same with Majora. It's all the mysteries behind the mask, which really makes it so scary. Lift those mysteries and you might get a disappointing and unsatisfying story, like the ending of LOST.
Though it seems that those mysteries around the new game won't be lifted in a "Zelda Direct". All they announced today was a Treehouse Live stream for June 14th. This supports my fears that there probably won't be any other Zelda news around the E3. No 30th Anniversary, no more Tri Force Heroes content and so on. Otherwise they would have scheduled a Direct before the livestream, but they really just want to show the game and nothing else.