The vocabulary is that of an evil lord who is thoroughly in his delirium, with a fairly strong language, bombastic formulas and sometimes rare words. Reading therefore requires a certain level of English to understand everything.
The set is beautifully presented, with a division into several well-structured parts, beautiful images (some from the cartoon, others made especially for the book), and small "handwritten" annotations added by Finn, Jake and Marceline in response to comments of "author".
In short, an excellent book for lovers of Adventure Time, provided they can read English.