As written language contains more complex syntax than spoken language, exposure to written language provides opportunities for children to experience language input different from everyday speech. We investigated the distribution and nature of relative clauses in three large developmental corpora: one of child-directed speech (targeted at pre-schoolers) and two of text written for children - namely, picture books targeted at pre-schoolers for shared reading and children's own reading books. Relative clauses were more common in both types of book language. Within text, relative clause usage increased with intended age, and was more frequent in nonfiction than fiction. The types of relative clause structures in text co-occurred with specific lexical properties, such as noun animacy and pronoun use. Book language provides unique access to grammar not easily encountered in speech. This has implications for the distributional lexical-syntactic features and associated discourse functions that children experience and, from this, consequences for language development.