For me, one of the most interesting things was to study Bill's personal copy of the "original manuscript" of the big book. Photographs of that complete manuscript were published a few years ago in "The Book That Started It All". Bill sent copies of that original draft of the big book, (via manuscript) to a couple hundred people asking for their feedback. He then made notations based on this feedback in his personal copy of the manuscript. His personal copy contains many hand written notes and corrections, many of which are not in Bill's handwriting.
One interesting addition was to Bill's story itself. Four paragraphs were added to Bill's own story after copies of the original manuscript were made. These paragraphs can be found hand written at the beginning of Bills personal copy of that manuscript. Interestingly, these additions to Bill's story (which are included in the first edition of the book) are not in Bill's handwriting.
We formed a group to study photos of that manuscript. It was quite interesting to hear the thoughts of others about why specific changes might have been made (revisions to the way Bill originally wrote it). There were quite a few changes in the final few months before the book came out in April of 1939.
"The Book That Started It All" is worth a look.