Answer: The book that was written about her father was no longer for sale, and the store owner, who had searched for the book, reluctantly turned her away.
Explanation: In the first sentence you would use the word 'that' because the book is not a person, and so the word 'who' would not be correct in that instance. In the second sentence, you would use the word 'who' because 'whom' is only used when the subject is receiving an action, not performing it. To combine the sentences, I used a comma and the word 'and' to make it a compound sentence.