We've just announced a four-part course I put together for Learn Squared, showing an intro to Houdini. This may be too basic for intermediate users, and probably not useful at all for power users, but for people just starting out and scratching their heads in confusion, it should be enlightening. We've put a lot of time into making sure the quality of these lessons is top-notch.
Learn Squared is a new way of doing these kinds of online classes. Each course offers mentorship critiques through skype sessions, and each apprentice's journey is recorded and documented as part of the class. We strive to get a lot of cross-discipline relationships started. For instance, I became an apprentice to a matte painter, and my first apprentice was a real-time graphics artist. There are multiple levels of entry with different price points, etc., so not everyone has to be an apprentice and document their work.
Check out the course here: https://www.learnsquared.com/courses/houdini-procedural-foundations
There's a lot of other great content there, too, which is useful across the CG and design spectrum.