other great tutorials on the internet

Houdini Tutotials - http://www.vfxhomeland.com/

Ben Watts - C4D and Houdini tutorials - http://benwattsdesign.com/

Particle Skull - Particle Flow and Houdini Tutorials - http://www.pskull.com/

Peter Quint's Houdini tutorials - https://vimeo.com/user2030228