The Standard ML Basis Library is the result of a decade of work by a small group of SML implementers.
The SML Basis Library Manual has been published in September 2004.
The book's page at Cambridge University Press.
The book's page at amazon.co.uk and at amazon.com.
The latest versions of Moscow ML, Standard ML of New Jersey, and MLton implement significant portions of the library.
Here are some online versions of the SML Basis Library documentation: