You've misunderstood -- works must have been copyrighted PRIOR to 1923 to be in the public domain. I'm sorry to say that Dorothy Sayers' books are still copyrighted, at least in the U.S.
This link to information at the U.S. copyright office can shed some light on this issue:
Scroll down to this paragraph:
Works Published and Copyrighted Before January 1, 1978 · A work published before January 1, 1978, and copyrighted within the past 75 years may still be protected by copyright in the United States if a valid renewal registration was made during the 28th year of the first term of the copyright. If renewed by registration or under the Copyright Renewal Act of 1992 and if still valid under the other provisions of the law, the copyright will expire 95 years from the end of the year in which it was first secured.
Therefore, the U.S. copyright in any work published or copyrighted prior to January 1, 1923, has expired by operation of law, and the work has permanently fallen into the public domain in the United States. For example, on January 1, 1997, copyrights in works first published or copyrighted before January 1, 1922, have expired; on January 1, 1998, copyrights in works first published or copyrighted before January 1, 1923, have expired. Unless the copyright law is changed again, no works under protection on January 1, 1999, will fall into the public domain in the United States until January 1, 2019.
Works First Published or Copyrighted Between January 1, 1923, and December 31, 1949, But Not Renewed · If a work was first published or copyrighted between January 1, 1923, and December 31, 1949, it is important to determine whether the copyright was renewed during the last (28th) year of the first term of the copyright. This can be done by searching the Copyright Office records or catalogs as explained previously. If no renewal registration was made, copyright protection expired permanently at the end of the 28th year of the year date it was first secured.
Works First Published or Copyrighted Between January 1, 1923, and December 31, 1949, and Registered for Renewal · When a valid renewal registration was made and copyright in the work was in its second term on December 31, 1977, the renewal copyright term was extended under the latest act to 67 years. In these cases, copyright will last for a total of 95 years from the end of the year in which copyright was originally secured. Example: Copyright in a work first published in 1925 and renewed in 1953 will expire on December 31, 2020.