Dear,
I’m not a kid anymore but I do like your book and cannot let it down since yesterday 
I downloaded the source codes twice or more but many folders are empty… what happens ?
Thank you so much for your work and kind regards from France
So glad to hear that you are enjoying the book!
The empty folders are leftover from the first edition of the book. Folders like advanced_avatar, appendix_tumblr, cave_levels, cave_puzzle, cave_puzzle_ii, console_for_fun_and_fixing, etc. were all chapters in the first edition that I either dropped or completely rewrote in this second edition. They should not have been included. This is probably a mistake that I made when I handed the code over to my publisher. Sorry for the confusion.
Hopefully that answers your question. If not, let me know. And kind regards to you from the US 
-Chris