Database: books
Question: What is the name of the publisher who published Agatha Christie's first book?
SQL: SELECT T4.publisher_name FROM book AS T1 INNER JOIN book_author AS T2 ON T1.book_id = T2.book_id INNER JOIN author AS T3 ON T3.author_id = T2.author_id INNER JOIN publisher AS T4 ON T4.publisher_id = T1.publisher_id WHERE T3.author_name = 'Agatha Christie' ORDER BY T1.publication_date ASC LIMIT 1