You've seen the handbook from the people who run Steam. Now, here's the company guide that employees at the House of Mario receive. It's a damn pretty thing to look at, even if you can't read Japanese.
Nintendo fan blog Before Mario has images of the internal publication that gets handed out to people who've accepted jobs at Nintendo. The design changes every year and, apparently, you still get one if you pass the first few stages of the intake process but don't get a job.
The 2014 edition takes visual inspiration from the company's past as a producer of hanafuda cards, with illustrations that portray old-school Nintendo toys in hand-drawn style. If you grew up with this history in your home, it must be like looking at your childhood.