The C64 Programmer's Reference Guide[1] was the game changer for me. 6510 instruction set, the computer's full memory map, register maps for all the chips, detailed information about I/O, the KERNAL, and so on. I don't recall it coming in the box, I had to save up my $$$ and actually buy the book.
Yes, and don't forget the great schematic diagram in the back! It definitely didn't come in the box with the C64; I had to save up as well. I still have my copy, tattered though it is, sitting here on my office bookshelf. That, and the book _Assembly Language Programming with the Commodore 64_ [1] changed the way I viewed computers.
1: https://www.c64-wiki.com/wiki/Commodore_64_Programmer%27s_Re...