Grosset & Dunlap, a major publisher of juvenile series books, adopted a new format for hardcover books with a color printed illustration on the cloth rather than a separate illustrated dust jacket in 1961 and 1962. These were cheaper to make, a bit more durable, and looked nice on the shelf.
It was also possible to create brand identity for each series by selecting a color for the spine of your pictorial cover. Thus yellow for Nancy Drew, blue for Hardy Boys, yellow-orange for Tom Swift Jr., gray for Rick Brant, purple for the Bobbsey Twins, and so on.
Grosset & Dunlap referred to these as “art boards” but collectors routinely call them “pictorial cover” formats.
In sales lists, this might be abbreviated as PC. It should not be done in computer listings on eBay and the like since this definition, unlike “DJ” for dust jacket, is not universally known. Readers are just as likely to assume it means the wider definition of “politically correct” and assume it refers to “original text”/”revised text” distinctions.
One disadvantage of the new format is that there is no longer a front and back flap on which to advertise. The rear panel is routinely used to list titles in the present series. Thus, cross-promotion of series was greatly curtailed.
Some series were slower to adopt the new format than others. A factor in this was existing warehouse stock of volumes in dust jacket editions. Thus, even when a series began to use the pictorial cover format, not all of the volumes were reprinted in it before the series was discontinued and out of print.
Only some of the 72 Bobbsey Twins titles are available in their purple spine PC editions.
Semi-advanced collectors of Judy Bolton may be frustrated to learn that a few volumes are not available in PC.
Similarly only a few Ken Holt or Connie Blair books are available in this latter format.
Conferring with other collectors, in person, online, or via series book reference books with format information is the only way to learn which are really available and which are the same as phantom titles in that format.