I thought if it's an object it becomes "ett"? And people/living things are en? I'm confused ;-;
"Ett" vs. "en" is largely arbitrary, I'm afraid. People and living things are usually "en", but for various historical reasons there are several exceptions: ett barn (a child), ett får (a sheep), ett lejon (a lion), ett djur (an animal), .. Non-living things can be either - you just have to learn. Around 75% of all Swedish nouns are "en", though, so that's a good bet if you have to guess :)
En/Ett doesn't depend on whether it's an object or not. The easiest way of learning which one to use is probably to learn it along with the word itself.
Does book have the same double meaning as in the object and the act of booking a hotel for example or is it different