There is a lot of material for learning Houdini now days, here is a few links
Matt has a great site with a bunch of snippets and great examples as well on how to get started
Side FX has a foundations book as well, Pretty well done.
Henry also has some great thing on his blog. Also he and Moritz are the creators of MOPs https://www.motionoperators.com/
Rohan has a series of introduction to Houdini. Which are pretty easy to understand.
Cg Circuit just release a series as well