The Kay Tracey series was published by the Cupples and Leon Company during the 1930s and 1940s.  From the 1950s through the early 1980s, the series was published by several different companies.  The series was written by four different authors who all shared the pseudonym of Frances K. Judd.  Most of the volumes, #3 through #12 as well as #14, were written by Mildred Wirt Benson.  #1 and #2 were written by Elizabeth M. Duffield Ward; #13 was written by Edna M. Stratemeyer Squier; and #15 through #18 were written by Anna Perot Rose Wright.

List of Titles in the Kay Tracey Series:

  1. The Secret of the Red Scarf, 1934
  2. The Strange Echo, 1934
  3. The Mystery of the Swaying Curtains, 1935
  4. The Shadow on the Door, 1935
  5. The Six-Fingered Glove Mystery, 1936
  6. The Green Cameo Mystery, 1936
  7. The Secret at the Windmill, 1937
  8. Beneath the Crimson Briar Bush, 1937
  9. The Message in the Sand Dunes, 1938
10. The Murmuring Portrait, 1938
11. When the Key Turned, 1939
12. In the Sunken Garden, 1939
13. The Forbidden Tower, 1940
14. The Sacred Feather, 1940
15. The Lone Footprint, 1941
16. The Double Disguise, 1941
17. The Mansion of Secrets, 1942
18. The Mysterious Neighbors, 1942

Formats:

The printing history and format changes for the Kay Tracey series are rather odd and a bit difficult to explain.  Here goes:

The books printed from 1934 until the early 1940s are thick yellow books with good quality paper and a glossy frontispiece illustration.  The dust jackets each had a question mark on the spine.  At least volumes 1 through 16 were printed in the thick format.  All 18 volumes were printed with the question mark dust jacket.  Later printings of the books with the question mark dust jackets are less thick and have poor quality paper.

During the mid-1940s, the dust jackets were changed to a solid color spine with a spine symbol in the middle of the spine, indicating some part of the mystery.  Each volume had the dust jacket printed in one of three colors:  yellow, blue, or red.  The dust jacket for each volume was printed only with the spine color designated for that volume, either yellow, blue, or red.  There seems to be no pattern for which color was selected for each volume; it appears to be random.  These books all have poor quality paper and plain frontispiece illustrations.  This style of dust jacket is more difficult to find than the question mark dust jackets.

After Cupples and Leon stopped printing the series, The Mystery of the Swaying Curtains, The Shadow on the Door, and The Forbidden Tower each went out of print permanently.

During the 1950s, 15 of the books (all except the three mentioned directly above) were reprinted by Garden City Books and Books, Inc.  Both sets have the same wraparound dust jackets.  The Garden City books were printed first and are shorter than the Books, Inc. books.  During the 1960s and 1970s, all 15 books were reprinted by Books, Inc. in a set of paperbacks and a set of picture covers that have the exact same cover art as the Garden City and Books, Inc. hardcover dust-jacketed sets.  This means that four different sets of Kay Tracey books have the same cover art.  The volume numbers are changed for these sets, so most of the volumes have different volume numbers than when printed by Cupples and Leon.

During the 1960s, 8 titles were reprinted by Berkley in paperback with new cover art and with the volumes in yet another different order.  The volumes printed in this set are Sacred Feather, Mansion of Secrets, Six Fingered Glove, Mysterious Neighbors, Green Cameo, Double Disguise, Strange Echo, and Sand Dunes.

During the late 1970s, a set of 6 books were reprinted with cover art that very closely resembles the Garden City and Books, Inc. cover art, but is actually new cover art redrawn to mimic the older cover art.  This set includes Mansion of Secrets, Six Fingered Glove, Double Disguise, Sunken Garden, Green Cameo, and Sand Dunes.

In 1980, a set of 6 paperbacks were printed by Bantam, also with new cover art.  This set includes Double Disguise, Sunken Garden, Six Fingered Glove, Mansion of Secrets, Green Cameo, and Sand Dunes.  

Texts:

For the most part, the texts remained unchanged when reprinted by each publisher.  The only three books that actually have significantly revised texts are Red Scarf, Green Cameo, and Strange Echo.  The Garden City and Books, Inc. texts of these three books are around 75% different from the original Cupples and Leon texts.  The remaining 12 texts that were reprinted by other publishers are either identical or nearly identical to the Cupples and Leon texts.  The only changes are a few stray words or passages, or possibly a character's name.