The term 'graphic novel' has not quite found its definition yet. It is more serious than a comic book, but more illustrated than The Grapes of Wrath. It is more than a book with pictures, since the Sports Illustrated swimsuit issue might qualify for that definition. It is a novel, a story with a plot line and character development, but much of the expression, the "telling," told with pictures. But in the present case, we have fewer illustrations than the term usually implies.
Did James Thurber write graphic stories? His 1939 The Last Flower surely qualifies as a graphic novel, perhaps the earliest example. Charles Dickens' works were virtually all illustrated from the get-go, with Great Expectations and Hard Times as the only exceptions, and these did not remain un-illustrated for long.
I take the term "graphic novel" to indicate a novel that is primarily told through its pictures, so I will choose to call Letters to Momma an illustrated novel.