Looking for suggestions on which book to read now

Hi I am a father for a chess boy and looking for readings/books to help him improve his calculation.

He is about 2200 rating in chess.com rapid and 2500 on chesscom puzzles now. Looking for effective ways to improve calculation.

2 books in in lists, both by Jacob Aagaard:

- Grandmaster preparation: calculation

- Grandmaster preparation: thinking inside the box.

Now hesitating which to read first.

Have seen different opposite views:

- Aargaard posted once about his GP series and saying "thinking inside the box" goes last as it connects everything together.

- A book reviewer on amazon, also a chess coach recommended this book goes before every other GP books as it will help get more from others. and wrote "Aagaard recommended so for a reason" which I cannot find the source.

The boy: his coach says he's good at position analysis and correct understanding. but his coach have no experience on above books.

I hear that pawn endgames are great for calculation too. I think that Karsten Mullet’s Secrets to Pawn Endings is supposed to be good, if difficult to get your hands on

Karsten Muller*, my autocorrect is playing tricks on me

I am a weak player, so I shouldn’t give my opinion here, but… I have several of Aagaard’s books, also “Calculation “. From time to time I take the book and go through one or two games. The examples are great, a lot of moves I wouldn’t play even in my dreams, like Sokolov vs Mitkov, black to play on move 17. The move wasn’t played in the game, it traps the Queen. Or game Mamedyarov vs Vallejo Pons, black to play one move 35. The move was not played in the game because it is insane, as black runs into a simple trap that turns to be good for black.
I think the book will help your son to get to next level, if he is ready to invest what is needed to absorb the material (this means, write down the analysis before checking the answer). But he will certainly like the book.

This review says the thinking inside the box book should be read first:

http://gollumchessreviews.blogspot.com/2017/10/review-thinking-inside-box-by-aagaard.html

Specifically it says "If I understood it correctly, this is the first book you are supposed to read in the series and will lay down the foundations of the work with the other books. It is like a synopsis plus a book on self-improvement. Aagaard tries hard to explain his approach to how do you improve on chess: making better decisions at the board. He classifies the decisions you make into four categories and tries to give you some tips for each of this decisions. Moreover you will get acquainted with the material of the other books on the series before you can try to solve puzzles (except for endgame play which is totally ignored here)."