The book analogy is most likely correct. He's saying if the "cover" wasn't there ("if I was see-through"), could people get to know the real him ("Could I be read") or would they just make a quick judgment and move on ( "Or would you just read my spine"). Much like reading the spine of a book to find out its title/author and guessing what the book is about, without actually reading what's inside it.
"Never judge a book by its cover"
The book analogy is most likely correct. He's saying if the "cover" wasn't there ("if I was see-through"), could people get to know the real him ("Could I be read") or would they just make a quick judgment and move on ( "Or would you just read my spine"). Much like reading the spine of a book to find out its title/author and guessing what the book is about, without actually reading what's inside it.