''When we stay up late and talk in the dark we always talk about Mum and Dad.'' The mysteries of a happy marriage, as seen through the innocent eyes of a child. Andrew Lindsay's book is a funny, wide-eyed, occasionally mischievous and ultimately touching tribute to parental love, in its many forms. ''Mum says that when she first saw Dad her heart stopped beating. I think they call it true love. Dad just says 'cool!'''