In Suruga Monkey Part 2 of Bakemonogatari, Kanbaru's room is literally full of red hardcover books. Why is this and what does it represent?
That's her massive collection of BL (boys' love) books (which she is also into, besides considering herself a lesbian), at least that's what they represent. As many other things in Monogatari's animation, it should be taken more symbolically than literally. Take for example Senjougahara's "arsenal" of stationery in episode 1 of Bakemonogatari, or even more obviously the constantly shifting, multiplying, growing, falling etc. pile of pencils in episode 6 of Nisemonogatari (you'll know when you get there :) ).