The right one is ON "The book is ON the desk" :D Over and Above are used when the things "don't touch each other" the book is phisically on the desk, it touches the desk, so it's used on :)

If the book was placed in a book shelf above the desk you can say : I will get my book from the bookshelf above the desk. Don't throw my book over the desk as the pages will get torn.

I hope these examples help. A book is floating above the desk. I am throwing book over the desk.

The book is on top of the desk. The book is on the desk. The book is under the desk.

The book is on the desk :=)