The Marchesi Cover books were each available for a period of about 4 - 5 years and most of the titles saw 8 or 9 print runs.   The books measure about 5-1/4" X 7-1/2" in size.  Early printings of the Marchesi Covers are noticeably thicker than later printings and this is because later printings use a thinner gauge of paper.  At the top of the spine is an oval containing the silhouette of  Alfred Hitchcock with the background color inside the oval being the same color as the spine (on other paperback formats the background color inside the oval is white).   Below Hitch is the title of the book.  Three Investigators paperback books printed in the years 1978 and 1979 do not have the title number printed on the spine.   As always, I recommend that serious collectors and/or investors pursue First Printings in the best possible condition.  One of the really nice things about the Marchesi books is that they each have the same internal illustrations that appeared in the original hardbound editions.  A collector should have little trouble obtaining each of the titles in this format.  By 1982 and 1983, each of these 12 titles was available in the new Hitchcock Cover format with cover art by Robert Adragna. 
Marchesi Cover, Hitch Cover, Keyhole Cover
Back to Three Investigators Paperback Editions/Formats.
In 1978, Random House began printing The Three Investigators series exclusively in a paperback format for the trade edition.  Artist Stephen Marchesi was chosen to create new cover art.  His work can also be seen in the large hardbound children's anthology "Alfred Hitchcock's Witch's Brew" from 1977 published by Random House.  Stephen Marchesi is still illustrating books today and you can learn more about him and his Three Investigators experience by clicking HERE to read an interview with him.  His abstract artistic contributions to The Three Investigators series are wonderfully unique and the Marchesi Cover books rank among the favorites of collectors worldwide.  Unfortunately, Marchesi's relationship with The Three Investigators series was rather short-lived possibly because of the abstract nature of his work. Up to this point, all Three Investigators art had been realistic in nature and after Marchesi's departure, Robert Adragna returned The Three Investigators covers to more reality-based cover art.  Because of soft-conversion manufacturing processes, Random House published only the following 12 titles with Marchesi cover art in 1978:

#1 The Secret of Terror Castle
#2 The Mystery of the Stuttering Parrot
#3 The Mystery of the Whispering Mummy
#6 The Secret of Skeleton Island
#7 The Mystery of the Fiery Eye
#8 The Mystery of the Silver Spider
#9 The Mystery of the Screaming Clock
#10 The Mystery of the Moaning Cave
#11 The Mystery of the Talking Skull
#12 The Mystery of the Laughing Shadow
#15 The Mystery of the Flaming Footprints
#18 The Mystery of the Shrinking House