Nowadays every issue of a comic book is a temporarily embarrassed graphic novel.
My definition of graphic novel: A book using original comics material in narrative. The important word here is book, so length isn't a criterion--"novel" encompasses collections (like A Contract with God, by the way). A magazine isn't a book, so His Name Is Savage, for instance, doesn't meet the definition. And if it isn't limited to original material, the floodgates open for an appalling number of titles from comic strips and comic books. Note that these three Pogo books are the original-material ones among, otherwise, collections. I say "in narrative" to omit single-page gags of the "talking in pictures" sort, although fumetti would otherwise fit--for instance, in the 70s, Richard J. Anobile's Fotonovels. (And so if we were including magazines, Charlton's 1963 full-length Black Zoo would predate the 1968 His Name Is Savage anyway.)
Mansion of Evil and It Rhymes with Lust are each dated just 1950, so which one is the first modern graphic novel is a question--but it's one or the other.
I'm pushing the definition of "original" with Passionella, but one of the four stores is original and the others have been revised and redrawn for the book. I dithered over the six 1962 Tintin books form Golden Press--they are new American translations--and decided not to include them. Belmont's High Camp Superheroes and My Son the Teenager I left off too, as the new material in them was prepared for the comics and by luck came out in the paperbacks first.
Can you think of any I'm missing through 1970? UPDATE--2 days later--The Monkees occurred to me when I happened to be thinking of the more usual TV tie-in comics and novels. FURTHER UPDATE: OtherEric in his comment contributed The Pogo Poop Book.
1940s-60s American Graphic Novels
abbreviations: mass market paperbacks, hardcovers, trade paperbacks
|1950||Mansion of Evil||Fawcett||pb|
|It Rhymes with Lust||St. John||digest|
|1953||Uncle Pogo So-So Stories||S&S||hc, tp|
|1954||The Pogo Stepmother Goose||S&S||tp|
|1955||The Pogo Peek-a-Book||S&S||hc, tp|
|1959||Passionella and Other Stories||McGraw-Hill||tp|
|Harvey Kurtzman's Jungle Book||Ballantine||pb|
|1960||The Flintstones on the Rocks||Dell||small tp|
|Huck and Yogi Jamboree||Dell||small tp|
|1962||Don Martin Steps Out||Signet||pb|
|[and many more MAD originals]|
|1966||Christopher Lee's Treasury of Terror||Pyramid||pb|
|The Monkees||Popular Library||pb
|The Great Society Comic Book||Parallax||small tp|
|Bobman and Teddy||Parallax||small tp|
|Kosher Comics||Parallax||small tp|
|The Pogo Poop Book||S&S||tp|
|1967||The Man from M.O.T.H.E.R.||Parallax||smaller tp|