Holding an open book enables an embracing composition of mother and child; the mother can also hold the child steady and prevent wriggling.
As cameras got smaller and faster, these exterior portraits become more naturalistic as the book is no longer required as a physical steadier. The position of the hands holding the book is more natural and suggests that the books are actually being read. In these portraits, the sitter has been given more freedom to choose their position and the books look as if they belong to the sitter, rather than being handed out as props to support a pose.
