Each of the first 23 titles in The Three Investigators series was originally published in hardbound with what is referred to as a matte picture cover. In the manufacturing process the cover art is printed onto matte paper, cut, and then wrapped and affixed to the hardboard covers. The Three Investigators series was never issued with a loose paper dust jacket in the U.S.
The matte cover was utilized from 1964 up through 1975 with book #23 "Invisible Dog" being the last to sport it. All hardbound trade edition books manufactured between 1976 and 1978 had a glossy picture cover beginning with book #24 "Death Trap Mine". One can obtain several of the first 23 titles in the glossy cover format (#'s 4, 5, 11, 13, 16, 17, 19, and 20) but these are books which were reprinted between 1975 and 1978.
In 1975, Random House discontinued the use of the blue graveyard endpapers in the books and began using plain white endpapers. It seems that the conversion of the endpapers took place very early in the first print run for #23 "Invisible Dog". Nearly all First Printings of this title contain the white endpapers but I have seen one copy which had the blue graveyard eps and I would consider any person fortunate to own one as they are apparently quite rare. In reality, what most likely happened was that some old endpaper stock was found during the printing of #23 Invisible Dog and simply used up at that time. I say this because the First Printings of #21 Haunted Mirror and #22 Dead Man's Riddle which both list to #22 on the back covers have the blue graveyard endpapers. Second Printings of both #21 and #22 also list to #22 the on the back covers but they contain plain WHITE endpapers whereas they SHOULD contain blue graveyard eps if they were printed prior to the publishing of #23 Invisible Dog. Ah well, one more mystery for us that will probably never be solved to our satisfaction. Please note, too, that most other matte cover books listing to #22 or to #23 on the back cover have the white eps but one can occasionally find an exception.
*There is one interesting printing error that occurred in the very early years of this series: "The Secret of Terror Castle" is incorrectly printed as "The Mystery of Terror Castle" on the back cover of some books. This misprint occurred in late 1965 up until it was corrected with the First Printings of titles #7 and #8. The misprint can be found on all First Printings of titles #5 and #6 (which list to #6 on the back cover) and any of the first four titles in the series which list either four or six titles on the back cover.
