Two Different Communication Genres and Implications for Vocabulary Development and Learning to Read: The abstract says it all for this one: you will find significantly more extensive and varied vocabulary in children's picture books than you will in child-led or even adult speech.

Study Says Reading Aloud to Children, More Than Talking, Builds Literacy
A different explanation of the same Massarro study above, but highlights the grammar mastery and reading acquisition benefits.

The Benefits of Read Alouds for Young Readers
This is an awesome compilation of research describing the benefits of reading aloud, as well as some effective strategies for reading aloud.

30 Key Literary Stats Parents Need to Be Aware of
These are some excellent points from a high-quality organization, based on sound research. It includes benefits of reading aloud, statistics on current child literacy, and the risks of not reading aloud.