I actually got Apple support to call me about the app automatically loading the "Reading Now" page every time I opened it. (And also to ask about making the book cover images smaller so that I could see more than 6 books on the screen at a time in my Library...)
Short answer: You cannot make "Reading Now" go away. And you cannot open directly to your Library.
Basically, Apple wants to force you to look at a bunch of ads every time you open the app. That is essentially what the "Want to Read" line and the other lines below that are on the "Reading Now" page. So, you, I, everyone else who uses the app will need to switch over to our libraries every time after we open the app. (And in my case, that means three presses on the screen once I'm in the app, just to get to where I want to be: Open app, press "Library" at the bottom of the page, then press "Collections" at the top of the page, and finally press "My Books" in the list of collections.)
And if you are interested in changing the display size in the library or anything else like that, you cannot do that either, if you were wondering.