A clear, thorough reference for the entire Standard Library, including the Standard Template Library (STL) plus strings, iostreams, and all the rest.  I've depended on Musser and Saini's STL Tutorial and Reference Guide for years, but Josuttis is clearly superior.  Some annoying gaps remain; for example, there's no obvious index entry to check when you want to know which operations on containers can invalidate iterators. At times, too, I long for more discussion of design: for example, it would be good to know <i>why</i> iostreams let you test a stream in a logical expression <code>if (in)</code> but STL iterators require the explicit comparison <code>iter!=myList.end()</code>.  But, for now, Josuttis is probably the best desktop reference.