As far I know, this isn't a feature.
Just like most android apps, it would be a nice addition being able to access the library swiping the left side of the screen.
Not necessarily as a sidebar, just "fading" the library ("full size") could work.
Why would anyone want that when there's a button right below ? Well, it's a bit frustrating when you're used to do it with every other app or come from other music players.