For Silman's complete endgame course, there is different sections for different ratings right?But what if I got an endgame not in the rating section for me? Can I look at it briefly or must zi reach the rating first?
Well, if you own the copy of a book you can do whatever you want with it, other than violate distribution rights. Peeking at later chapters certainly is an option.
Honestly though, I'd only buy Silman to follow Silman's structure. All endgame books have more or less the same content, essentially it's the structure + selection of paedagogical examples that you buy.
The progression of Silman's chapters is intended for chapter-after-chapter study, so if you go with Silman, for your learning, follow that structure. If you are referring to using reference material for correspondence games, i.e. want to read about a specific endgame which occured in your games, then sure go to the right chapter but tbh Silman's structure is not best fit for reference, it's mostly the learning curve that is unique there. Perhaps get Silman + another book ( Keres or Karsten Mueller for reference )?
For "real" reference, in the old days, folks were using Averbakh's books but my view is that those Averbakh tomes is the perfect way to turn endgames into a boring topic.
I*