I have books that are not at the current version of WordPress, but they might still be of some value. If I learned to develop on 3.5 or 4.0, for example, but later on 4.x+, would it be completely different to work on the newest version?
This is a pretty broad question, but I will still try to answer it.
WordPress is a mature platform so you won't see many changes between 3.5 and the current version that would require you to learn from scratch. The basics of WordPress are still the same with some additional features that will make your life easier added since 3.5. You might stumble upon a function call or action/filter hook that has been deprecated, but there aren't many of them and, as long as you develop with
WP_DEBUG enabled you will be fine.
WordPress books or online courses are a good starting point to get a grasp of basics (how the loop works, templating hierarchy etc.) and these haven't changed. Beyond that, WordPress Codex is your best friend and, regardless of WordPress version, you will spend most of your time there. Just remember to always use/develop the most recent version of WordPress.