New users are encouraged to start by looking at the Solr tutorial.
In each Solr binary release artifact, you can find a copy of the Solr tutorial, a detailed list of changes, and JavaDocs for all included code.
Copies of this documentation for the most recent release of each major branch under active development can also be found online:
The Apache Solr Reference Guide
Beginning with Solr 4.4, A detailed reference guide is also available as a PDF download for each minor release.
Archives for all past versions of the Solr Reference Guide are available at the Apache archives.