The only time Jason has previously worked on a book that wasn't entirely written by himself was The Iron Wagon, his comics adaptation of a novel. Isle of 100,000 Graves represents his first true collaboration, with writer Fabien Vehlmann providing a template that is a remarkable complement to Jason's deadpan style. It's a genre story (concerning pirates, treasure maps, torturers, and deadly secrets) whose success hinges on the emotional beats of its main characters and the eventual relationship that ensues between its leads. At a crisp 57 pages, Vehlmann and Jason cram a surprising amount of plot and character development into this graphic novella, yet the book has a pleasantly unhurried pace and plenty of room for gags. Vehlmann doesn't quite nail down the essentially melancholy tone of Jason's writing style, and as deadpan as he gets, the story is still a bit more verbally effusive than the standard Jason book.
That said, the story of Gwenny, the scrappy girl who manages to fool a group of pirates into helping her find her father (with the help of a treasure map), is tinged with a sense of repressed sadness and rage. Gwenny's story begins when she finds a map to the titular isle and is suddenly placed in harm's way, starting with her insane mother's attempt to murder her. Throughout the book, the clever girl is one step ahead of her opponents while never showing her hand. She tricks a pirate into being her personal bodyguard by whispering "I know your secret" to different men, until one actually responds. She's preternaturally resourceful and her world-weary cynicism proves itself to be repeatedly prescient. The only problem with the character is that we don't get a real sense of her emotional core until the very end of the book. Jason usually provides a bit more context as to his characters' feelings a bit earlier in his books, usually by way of showing the reader seemingly mundane details from their everyday lives.
The other protagonist turns out to be a boy named Tobias, a student at the "hangman's academy" located on the isle who isn't very good at his studies. When this particular plot twist is revealed (and it's deliberately telegraphed on the third page of the story), the real comedic core of the book is unleashed. Vehlmann spends a number of pages turning the book into a workplace comedy, with the twist being that the workplace is a torturer's academy that lures greedy explorers onto the island thanks to the "treasure map" that's sent floating in bottles by the thousand. There are all sorts of funny beats to be found as the headmaster goes on tangents about the lost art and honor of torture in a mechanical age, the teachers cluck about Tobias' ineptitude in scholastic areas such as strangling and pyre-making, and the guards laugh at Gwenny's attempt at sneaking in as a technique they've seen time and again. The shy, nervous Tobias is the most compelling character in the book, and his instantly falling in love with Gwenny gives the book the charge that propels it through its satisfying back half.
The secret hero of this book's success is the colorist Hubert, who brings a vivid richness to the book that gives it a quality not unlike that of Carl Barks' work. While that palette is kept simple on a per-page basis, Hubert changes up the colors from page to page in a manner that keeps the reader engaged with the material's shifting emotional tones and narrative twists. The simplicity of his palette is a perfect match with the simplicity of Jason's clear-line, anthropomorphic style. The result is pure storytelling pleasure, a kind of narrative eye-candy that is doubly attractive for its sense of restraint and Vehlmann's deadpan story beats. All told, I wouldn't say that this book matches up with Jason's top-rank work (I'd include I Killed Adolf Hitler, Hey, Wait..., and The Left Bank Gang as the best of the best), but the collaboration with Vehlmann was clearly quite fruitful for both artists. I'd be interested in seeing the pair collaborate again to see how getting one project under their belts might lead both artists to getting more in tune with the other.