The Twyford Code by Janice Hallett

When Steve was a child, a favorite teacher of his named Miss Iles disappeared on a school field trip. Now an adult and just out of prison after a long stretch, Steve decides to delve into his past to find out what happened to her. Steve believes the mystery of  her vanishing is tied to a book he brought to class one day by children's author Edith Twyford. With his estranged son's old i-phone, Steve records his conversations with his classmates from Miss Iles' class, a librarian named Lucy who helps Steve, and other people he encounters on his journey. With a colorful main character and a puzzle that keeps the reader guessing until the end, The Twyford Code is a good read-alike for the Thursday Murder Club series and Exit by Belinda Bauer.