Hi,
I don’t know if it’s the appropriate place to make a feedback regarding the books, if not, feel free to delete this post.
I’m using preview with the pdf ebooks from Ray Wenderlich (it’s not specific to swift apprentice), and I think it would be very useful to be able to use the “dynamic” table of contents to navigate within the book; it’ll help to jump from one topic to an other without having going each time at the beginning of the book to identify the right section, then finding the section of interest with the page number, and so on. I admit, I’m lazy and I love saving time 
But since it’s an available functionality, I’d love to enjoy it.
Thank you