question about a sentence
1. what does " it bore the unmistakable JFK stamp"mean?
2.and in "which could be discerned in Kennedy’s first book", does this a book about Knnedy written by Ted Sorensen, or a book written by Kennedy?

context:
【They had been bouncing drafts back and forth for a week. It was an old routine, and although the result was joint authorship it bore the unmistakable JFK stamp, for Ted had adopted the literary style which could be discerned in Kennedy’s first book, published thirteen years before the two men met. 】

Question

question about a sentence
1. what does " it bore the unmistakable JFK stamp"mean? 
2.and in "which could be discerned in Kennedy’s first book", does this a book about Knnedy written by Ted Sorensen, or a book written by Kennedy?

context:
【They had been bouncing drafts back and forth for a week. It was an old routine, and although the result was joint authorship it bore the unmistakable JFK stamp, for Ted had adopted the literary style which could be discerned in Kennedy’s first book, published thirteen years before the two men met. 】

Answer

1. unmistakable JFK stamp - its style of writing had characteristics of the style of writing JFK used in books which he wrote

2. which could be discerned in Kennedy's first book - which gould be seen in the first book which Kennedy (JFK) wrote.  In the book co-authored by Ted Sorenson and Ted Kennedy, you could find similarities between the writing style in their book and the first book that John F. Kennedy wrote (which was written long before Ted Sorenson met Ted Kennedy).