Like others have said, intrigue can be good. Do readers need to immediately know all the details of what happened to appreciate why the book signing is interesting? It sounds like something that could be fairly self-explanatory--the situation indicates that people 1) know about supernatural creatures in this world and 2) that it's still something that's novel and exciting for them. The reader doesn't need all the info right away. They just need to get an idea of what type of world this is, who the main character is, and what matters to them.
It might be that some of the back story is really important to the plot, but in that case, you might still be better off revealing it a bit at a time to allow for surprises and new developments. In the Harry Potter books, for example, Rowling could have laid out the entire story of what happened to Harry's parents in the first book. But it made sense that a child who didn't even know his parents were wizards wouldn't learn all the details at once, and developments like discovering who Sirius Black was were a major part of the plot that were more meaningful than if we were just told everything from the start.
But in terms of worldbuilding, it's often not essential for readers to know everything, especially not at first. How the world is affected now by what happened 50 years ago might be more immediately interesting, actually.