Buy once. Free updates as the book grows.
The book is free to read online. No account, no paywall, no drip-feed. Just read it.
The slides, cheatsheets, and digital PDF are there if you want something to keep offline or want to support the writing while it's still in progress. That's it.
If you're not big on long-form theory, the slides are worth it. More focused, illustration-heavy, and built for a different pace — same depth, less prose.