The Ghost and the Haunted Mansion by Alice Kimberly

This is the latest book in the Haunted Bookshop series featuring female-sleuth Penelope Thornton-McClure. The opening chapters find Penelope on her way to Miss Timothea Todd's mansion home on Larchmont Avenue. Larchmont Avenue is home to many of the wealthy in Penelope's hometown of Quindicott, Rhode Island.

Penelope is co-owner of the local bookstore "Buy the Book" which specializes in mysteries. They have recently started to sell books on the occult.

Penelope is going to deliver several occult-related books to Miss Todd, who is a shut-in. On the way there Penelope is startled to see her friend and postal worker Seymour Tarnish dashing through the neighborhood with a red spot on his shirt. When Penelope arrives at Miss Todd's home she finds the door wide open and the foyer table upended with mail scattered all over the foyer floor. She enters the home to find Miss Todd dead with a terrified expression on her face as if she had been frightened to death.

Penelope calls the local police and upon questioning, she mentions seeing Seymour. They pick him up for questioning and think he has killed Miss Todd because of the red spot on his shirt. It turns out the red spot is not blood, it's pizza sauce! Seymour had become friendly with Miss Todd and had bought some pizza slices to share with her for lunch that day. But they never had lunch since Miss Todd did not answer the door when Seymour rang it - she had unlocked the door, so he just set the mail on the foyer table and left. He explains he went out to the yard and sat under a tree, ate his pizza and spilled some on himself. He must have dozed off because when he woke up he was late for his mail route and that's why Penelope found him dashing through the neighborhood. He also thinks he forgot to close the front door tightly and it blew open from the wind.

All very plausible explanations but the police still suspect him, especially when they find out Miss Todd left him something in her recently-revised will!