Plato thought the bird to be a metaphor of the human mind and spirit, due to its weightlessness, ability to fly and reach the sky.  Reaching the sky was the divine symbol, and that is why in Christian tradition, the white dove is a symbol of the Holy Spirit.

White is associated with the light of truth and revelation, and with joy and celebration.

The book is a symbol of knowledge and wisdom with the pages of the book, pages of  human life; the maps we travel through, the pages written into history, the diary of  human life.

The book evokes imagination to which there is no limit.  The moment that imagination takes us from the usual everyday things to the distant spaces is the moment of discovery and truth, revelation.  In the painting, this is the moment of turning the page into the bird.  The line of  transformation is the trajectory of the bird's flight.

Reference: Snow Birds, White Flowers of the Sky.