Boost is a excellent resource for C++ programming, but suffers from inconsistent documentation and a daunting array of sub-projects . Trying to make sense of it all is a fairly serious undertaking. I tried to get my head around it by writing my occasional series of boost-related blog posts but now I see that somebody has done a much better job.
The Boost C++ Libraries is a free book that clearly explains some of the more generally useful boost libraries, with lots of useful examples. It even covers advanced libraries like ASIO in an approachable way. I highly recommended bookmarking it if you do any C++ programming.