Beginning with a Bash, by Phoebe Atwood Taylor (writing as Alice Tilton) (epub/Kindle/pdf)
The sign said COME IN AND BROWSE—IT’S WARM INSIDE. Martin Jones made the mistake of his life when he joined the others inside the bookstore. One of them, Professor John North, would soon be dead. And Martin is going to be the prime suspect.
Luckily for Martin the bookstore is owned by his friend Dot Peters, who knows he's innocent. Even with the able assistance of Leonidas Xenophon "Bill" Witherall, the retired headmaster of the Boston academy which Martin once attended, can she prove his innocence?