It’s a series of books called The Secret Series by Pseudonymous Bosch aka Raphael Simon.
In The Name of This Book Is Secret, a real estate agent for the deceased named Gloria finds a mysterious box called "The Symphony of Smells" in a dead magician's house, which she drops off at Cass' grandfather's junk store. Cass takes the Symphony of Smells to school with her the next day. That day, while she investigates the reason for a rat dying in her schoolyard, she meets Max-Ernest, who talks too much, loves jokes, and has divorced parents. Max-Ernest tries out one of his jokes on Cass, who tells him that his joke doesn't make sense, thus fostering a conversation between the two.
Cass shows Max-Ernest the Symphony of Smells, and they decode a message for help hidden inside it. Cass and Max-Ernest decide to visit the dead magician's house to find out why he needed help. Now collaborators, Cass and Max-Ernest go to investigate the dead magician's house and find the magician's mysterious journal hidden in his secret study. However, Gloria comes in with a young couple looking for a house - Ms. Mauvais and Dr. L. This couple is looking for the magician's journal and when they see the kids leaving with it, they follow Cass and Max-Ernest and utter threats for hours, however the kids end up escaping.
Quote from the book:
After her experience with Amber and Veronica, Cass had
vowed never to discuss her predictions with anyone again.
But she made an exception for Max-Ernest because he
seemed so knowledgeable about toxic waste. By the time
they returned to the soccer field with the laboratory gloves,
Cass had told him all about the dead magician, the dead
mouse, and the mysterious sulfur smell.