In my workshops people usually start being able to apply it to their business problems by about day 3. It's actually pretty simple once you get the hang of it!
After those two, I'd probably recommend the original _Specifying Systems_. It can be a tough read, but it's a comprehensive dive into how TLA+ works mathematically and everything that you can do with it. You can get it for free here: https://lamport.azurewebsites.net/tla/book.html
I'd also recommend practicing by applying it to problems you've already solved in your place of work. That way you can compare it against a real system you know inside and out. I talk more about that here: https://www.hillelwayne.com/post/using-formal-methods/
After those two, I'd probably recommend the original _Specifying Systems_. It can be a tough read, but it's a comprehensive dive into how TLA+ works mathematically and everything that you can do with it. You can get it for free here: https://lamport.azurewebsites.net/tla/book.html
I'd also recommend practicing by applying it to problems you've already solved in your place of work. That way you can compare it against a real system you know inside and out. I talk more about that here: https://www.hillelwayne.com/post/using-formal-methods/
Does that answer your question?