The set consists of eight books.
1. The Mystery of Morgan Castle, 1962
2. The Mystery of the Marble Angel, 1962
3. The Mystery of the Midnight Visitor, 1962
4. The Mystery of the Missing Marlin, 1962
5. The Mystery of the Musical Ghost, 1965
6. The Mystery of Monks' Island, 1965
7. The Mystery of the Marauder’s Gold, 1965
8. The Mystery of the Myrmidon’s Journey, 1965
The books run around 80 pages with large text. The first few books are very short, since they are for the youngest readers. The reading level is one grade higher in the later books. The print is smaller in the later books, so the stories are longer. The reading level of the last book is very close to the average series book. The book is still shorter than most all series books, but it is longer than the earlier books. Below are examples from early in the series and late in the series. Click on each image to see a larger version.
The books are best read in order. While the books can be read out of order, the reader will best appreciate the books if read in order. This is because the books have very little explanatory information, and the set of characters does build during the first few books. If the first few books have not been read, the reader will know none of that information because none of it is repeated in later books.
The parents are completely missing in these books due them conveniently being away from home. Sometimes the parents are around, but they are never present in any scenes. The young people occasionally call home to tell their parents what is going on, but the parents' responses are never included.
Series books typically have a mentor adult who is present. This person is sometimes a chaperone, but they are always indulgent with the sleuths and their adventures. Mrs. Wellington, who is introduced at the beginning of the series, is the primary mentor adult present in the books. A teacher from the boys' school, Mr. Ballard, also serves as a mentor adult in this series.
In general, the later titles are the best in the series, particularly Missing Marlin, Monks' Island, and Marauder's Gold.
Some of the books would have been outstanding series books if they had been fleshed out into full-length books. Even as short readers, they are still quite good.
Taken altogether, this set of books is like one series book. Each book is like a small story arc within a larger story.