Gargantua is a super massive black hole.

Spaghettification is caused by the gravitational force experienced at your head being different than the gravitational force experienced at your feet. A black hole formed by a dying star is in the neighborhood of 30km in radius. With a small black hole like that the differences in gravity experience between your head and toes is what leads to the spaghettification.

That's not the case with a supermassive black hole like the one at the center of our galaxy which is the kind Gargantua was supposed to be. Those can be as large as the sun (700,000km), and that's on the small end. Some can even be the size of our entire solar system. Because of their enormous size, the gravitation difference you'd experience between your head and toes is negligible.

And as far as navigating inside the black hole goes, that's actually what Roy Kerr says - the guy that solved the Einstein field equation for a rotating black hole. He says that in the case of a rotating black hole, the singularity at the center is not a single point but a ring. And because of that, spacetime is warped in a way that that would make it navigable. You wouldn't be able to leave (move back up past the event horizon), but you would be able to stay inside and have some effect on your location through your own force. At least that what he says. I'll take his word for it since he's spent a few decades studying it.

As for the bookshelf.... yeah, sorry. Science doesn't say shit about bookshelves inside a black hole. But, you know, it's a movie. It's probably a metaphor or some shit. But Christopher Nolan did a surprisingly good job of being accurate with the theoretical science of a black hole. Feel free to continue to hate the movie for saying love is a fundamental force though.

Sources: https://en.m.wikipedia.org/wiki/Supermassive_black_hole

https://www.youtube.com/watch?v=HRir6-9tsJs&t=10m22s