If the books are all the same size, lined up straight and even then they're just another wall. If the books are different sizes, lined up unevenly, a few here stuck further in, a few sticking out, then you've turned them into a diffuser panel.
A couch along the back wall, anything along a wall is going to have tremendous bass and you will probably need the traps but even that won't do it. Along the wall seating is a terrible idea if you'll use speakers with good bass. On the other hand could be just the ticket to reinforce leaner smaller speakers. Point being you weren't aware of this, won't know until you try it, so don't throw money away on tube traps until you're sure.
That size room with carpeting and bookshelves will be pretty quiet. Most of the noise will come in through the door. (And a surprising amount will come in through the window, even when closed.) But mostly the door. Ideally you would replace the normal hollow core indoor door with a solid core outer door and use weather stripping same as if it was an entry door. At the very least do a good job weather stripping your door. You'll be shocked how much noise comes in from even a small crack or gap.
These are the lowest cost easiest to do things that will have a huge impact. Beyond this you can easily spend big money stripping the walls and rebuilding from the studs out. Search around there's a guy here who did that to remodel a room for his Tekton Moabs in a room about the same size as yours. A huge project but he is very happy with the results.