Apache Solr PMC announced that the first official documentation for Apache Solr 4.4 (more about that version) is now available. The PDF file with documentation is available at: https://www.apache.org/dyn/closer.cgi/lucene/solr/ref-guide/.
It is worth mentioning that the original documentation was donated by Lucid Works and now it is a part of Solr project and will be maintained by the community.