Which books are told in the most interesting / creative/ mind bending ways? How does it add to the book overall?

My all time favourite is Ella Minnow Pea where the book is a series of letters. The characters have to think of more inventive ways to write their letters as an increasing number of letters are outlawed as the book progresses.

Honourable mentions include:

Maribou Stork Nightmares where the narrator is trying to suppress his dark past by allowing himself to slip into hallucinations of a whacky south African safari adventure.

Flowers for Algernon where the narrator becomes more articulate by taking part in a scientific experiment.