Ask HN: What is the best money you have spent on professional development?

You could try and see if you are allowed to spend the money on hours to work on a side project. If there's something you really feel like doing, but never get time to do, you could maybe spend a week working on that. Because it's a passion project it'll probably bring you more than any class can do.

An internet connection

for me it was linux academy (paid by my employer of the time) and getting red hat certified (rhcsa).

in order to get rhcsa certified i had to study and learn all of those little details about linux that i had only skimmed previously, and learn some things that i didn't even knew existed (ex: filesystems acl).

Getting Red Hat certified definitely made me a better sysadmin and a devops engineer with a stronger linux background.

edit: since moving on from that employer i'm paying for linuxacademy myself, although nowadays it's not clear what's going to happen when ther merger with acloudguru will be fully done.

As many others have said, books are the best things I've spent money on, but let me say more:

Do books sometimes say things that are obvious? Yes. For example, when I first read Martin Fowler's book Refactoring, I had been renaming variables and moving methods from one class to another for years. But he gave a new framework for thinking about something that obvious. I've found the idea of separating out my coding flow between adding functionality and improving code really helpful.

Can you get the material for free? Almost always, yes. But it takes time to find the right material, and our time is valuable. A typical industry book is $40-$50, and a typical text book is $100-$150. The authors of these books have spent time organizing the material in a helpful way that you would otherwise need to spend.

For some topics though, it's not just a matter of time savings. I'm working on a topic now that doesn't really have many useful books, so I'm having to read technical specifications produced by industry groups, which lack context and are pretty opaque. I'm missing having a book that explains these ideas in a coherent fashion.

I have tried a few MOOCs for professional development. I've found they can be helpful for a superficial understanding, but they don't encourage the deep understanding I get from reading through a book.

Right? I too failed at a self-funded startup. Most expensive 'mistake' of my life, but boy did I learn a lot.