If you want the closest match to Pip's approach to solving mysteries, start with Truly Devious or The Agathas.
Both feature teenage investigators who question accepted explanations and pursue evidence that others have overlooked. Truly Devious is especially appealing if you enjoy complicated clues and a mystery unfolding over several books, while The Agathas is a good choice if you prefer an investigative partnership.
If true-crime research is what captured your attention, try The Amateurs. Its young protagonists take their interest in unsolved cases beyond discussion and into an investigation of their own.
For readers who liked the secrets surrounding Pip's classmates and neighbors, One of Us Is Lying offers a different perspective: its main characters are themselves suspects in a murder.
Choose The Cheerleaders if you want an emotionally darker small-town mystery involving an old tragedy.
If the later books in Pip's series left you looking for more intense suspense, None Shall Sleep or The Naturals may be better choices. Both involve young people confronting dangerous criminals, but their investigative settings are very different.
The best choice depends on whether you most enjoyed Pip's detective work, the hidden secrets of her town, or the danger that followed her search for answers.