The documentation for all versions of DSpace is included with the source code, when you download the software at SourceForge: http://sourceforge.net/project/showfiles.php?group_id=19984

The documentation for 1.5.1  is available in PDF and HTML.  The HTML documentation is also in Sourceforge.  

The 1.5.1 release contains numerous bug fixes from the 1.5.0 release, including: browsing, display, and workflow issues in XMLUI; search result and submissions issues in JSPUI; Oracle compatibility; and some minor stability corrections.  In addition, the University of Minho statistics package will now work with XMLUI as well as JSPUI.

How-To Guides and Community-Contributed Guides

Members of the DSpace community have been very generous in adding their own how-to guides, translations, and customization documentation. For additional developer doc, customization guides, and other documentation contributed by the DSpace community, visit the DSpace wiki: http://wiki.dspace.org/index.php/DspaceResources#Documentation_and_Guides