Published yesterday, July 16th, the preliminary MSDN documentation for SharePoint 2013 is now available: http://msdn.microsoft.com/en-us/library/jj162979(office.15).aspx.
The SharePoint Server 2013 preview can be downloaded from TechNet at http://technet.microsoft.com/en-US/evalcenter/hh973397.
Microsoft’s guide to setting up a testing environment can be found here: http://msdn.microsoft.com/en-us/library/sharepoint/ee554869(v=office.15).
Happy Coding!