What can I do with my second bedroom to make it a home office/ library area?

My husband and I live in a two bedroom apartment, have no children and rarely have guests that need to spend the night so we're using the second bedroom as an office/library area. The problem is it hasn't come together very well. It technically functions for what we want but nothing is very concise about it. It feels like leftovers that didn't fit anywhere else. What can we do to fix it without spending much money? Preferably without spending any but I'm not sure that's possible. Because we're renting painting isn't an option but we can hang pictures. The carpet is the typical bland beige and the door and baseboard are I believe referred to as honey oak in color. The furniture consists of three small bookcases, waist high at best, a papasan chair and the (broken) computer chair and computer cart I'm currently using. The decor has a dragon and unicorn theme to it. Mostly by accident since we both had some from when we were teens and when we moved in together the office/library seemed to be the only place that made sense for it since he's a gamer and I like fantasy novels. We're in our mid twenties so I'm okay with that theme idea but don't want it either too young or else dark/creepy. I was thinking something like fantasy forest so I can work with what we have but bring in more nature elements but I don't know how to do it. It would also be nice if the room could function as a bedroom on the rare occasion it's needed but the only furniture I can think of for that is a futon and I'd really rather not have one of those. For one I think there's not enough space and two I think they're cheap and ugly. Does anyone have any good suggestions? I've tried looking online but most ideas are for houses so they involve things I can't do or they are the type of office used for business purposes which this clearly is not. There has to be a way for this room to be what we want it to be but I can't seem to find it. Help!