A whole half of the room for a bookshelf is pretty extreme, of course. But before you start looking for new storage spots, it's worth thinking about whether you actually need all these books. Half a kilometer worth is a lot, and you can definitely give some away, sell them, or just toss them. Paper books take up a huge amount of space for minimal information density. Keeping only what you actually need or love - that'll free up tons of space.
If you really do need to keep some books, it makes sense to use vertical space smartly. Hanging shelves above doors, along narrow walls, over a desk - that all helps a bit, but it's not magic. Built-in cabinets need renovation work, which is time-consuming and expensive. But shelving units in corners, especially corner models, take up less usable floor space than a rectangle in the middle of the room. The space between the ceiling and the wall works too - if your ceilings are high enough, a narrow shelf tucked up near the top is fine for holding books you rarely pull down.
Honestly, the real solution isn't new furniture, it's going through your collection with a critical eye. If you cut it down to half, the whole storage problem becomes way easier. What's left will be much simpler to fit in without messing up how you actually live in the room.