The Definition of Standard ML (Revised) is a terse and formal specification of Standard ML's syntax and semantics. The language specified by this book is often referred to as SML 97.
The Definition of Standard ML is an older version of the definition, published in 1990, which has an accompanying Commentary on Standard ML that introduces and explains the notation and approach. The same notation is used in the SML 97 definition, so it is worth purchasing the older definition and commentary if you intend a close study of the definition.