Your puzzle is on page 36 of the book. Open one hint at a time: often the first one is enough.
Each group of three numbers is: paragraph, word, letter. Count the paragraphs of Elias's letter from the top ("My dear friend," is paragraph 1).
Check your answer
Nothing is revealed if you are wrong.
First hint
Paragraph 1 is the greeting "My dear friend,". Count words left to right in each paragraph.
Second hint
The first group points to paragraph, word and letter: count carefully and write down the letter.
Solution
Counting paragraph, word and letter in Elias's letter: 2-26-4 = C, 3-20-1 = I, 4-1-2 = N, 5-8-4 = N, 4-3-3 = A, 3-14-1 = M, 2-18-2 = O, 4-14-2 = N. The answer is CINNAMON.
The same hints and solutions are printed in the book (pages 56, 58 and 60).