Reddit mentions of The Magic of Houdini

aw geez, most stuff I've used to learn Houdini must be obsolete by now.

There is this more or less up-to-date introductory tutorial. I haven't watched it, but it doesn't seem to dwell too much into the procedural nature of the software, which is the most important aspect of Houdini.

I'd recommend first to watch the first chapters of these old free lessons. while it is obsolete, it will give you basic notions of how Houdini works.

I started with this book, which patiently explains the basics with awesome easy to follow examples. I think most of its examples still work but you'd need to skip dynamics and particles as it changed a lot since 2005.

But be ready to fall into a rabbit hole, the possibilities are limitless and it is get easy to get overwhelmed and lost. The best thing is to know what you want to know/do and specialize rather than trying to master everything Houdini is able to do.