This book which is often studied in English class seems to effectively combine a series of different genres to create a compelling read. As a young adult book, there is of course the genre of the adventure novel that uses considerable humour, but at the same time, critics have also noted the elaborate plot structure of the novel that presents the reader with a puzzle that needs to be worked out. This is something that borrows from the genre of the detective novel. Also, there are very strong elements of the fairy tale within this text, especially in the way that the gypsy curse is so prominent and how two main characters meet who are actually related to each other through their ancestors. Note how these two individuals are connected with other events in the plot in the final chapter, that attempts, as the title suggests to "fill in" some of the holes that remain in terms of plot and understanding:
The reader might find it interesting, however, that Stanley's father invented his cure for foot odour the day after the great-great-grandson of Elya Yelnats carried the great-great-great-grandson of Madame Zeroni up the mountain.
Such connections are very common in fairy tales, and so there is a sense in which Sachar is borrowing from a number of different genres in writing this story and creating his tale.
Although Holes may not have an obvious specific genre, it does borrow from many different types of writing. Humor, suspense, action, foreshadowing, flashbacks are all included in the wonderful novel. The above answer mentions it as a fairy tale. A fairy tale is certainly appropriate for this type of novel, especially because of the way the characters are portrayed (good vs evil) and the way the book ends. Another possible classification for it may be a detective novel, just because of how the book unfolds. Either way, no matter how it may be defined, Holes makes for a great read that shouldn't go unnoticed by any bookworm.