The most direct answer to you question is that the "illustration" is a version of Arabic Calligraphic text: Kufic Script. The geometrically ornate square is aesthetically appealling on its own, but it also spells out the title of the translated book "Kitab Al Layla Wa Layla" (The book of a Thousand nights and one night) in each of four interlocking hatchet shaped figures within the large square as illustrated in the garishly coloured figure :). Reading from right to left : starting from the bottom right and proceeding leftward down the length of the hatchet handle then moving above to the blade section reads Kitab alf laylah wa laylah- proceeding clockwise this is repeated thrice over.
A far more detailed and lengthy exposition of implied significance of the figure with respect to aspects of its interpretation in relation to the books contents can be found in the preface and acknowledgements of Peter L. Caracciolo's: The Arabian Nights in English Literature Studies in the reception of The Thousand and One nights into British Culture: Macmillan 1988 if you are seeking a more elaboration than my own concise attempt at explaining "the illustration".