I put off buying a real bookcase for ages because every one I looked at online seemed like it was either going to cost as much as a used car or fall apart if I looked at it wrong. What changed my mind was unpacking the fourth box of books onto the floor of the office and realizing stacking them against the wall was not, in fact, a long term system.
If you have none right now, start with one tall, plain bookcase in a single room, probably the one where you actually sit and read or where the clutter piles up worst. Don't try to solve every room's storage with one trip. Get one good one, live with it, see what you actually need more of, whether that's more shelves, more width, or a second one entirely. I made the mistake of buying three cheap ones at once for three different rooms and ended up with three slightly wobbly regrets instead of one solid win.
Where it shouldn't go is anywhere already busy. A heavily patterned room, a wall with a gallery of frames, a space that already has a big armoire or media unit doing visual work. Bookcases want to either blend into the wall color and go quiet, or stand out as the one tall piece in an otherwise simple room. Put one in a room that's already loud and it just adds to the noise, and you stop noticing any of it, including the books.
The difference between a good bookcase and a disappointing one shows up fast once there's weight on the shelves. Cheap ones are usually particleboard with a printed wood veneer, held together with cam locks and glue, and the back panel is a sheet of cardboard stapled on. Load it with hardcovers and you'll see the middle shelves bow within a year, sometimes less. A well made one uses real plywood or solid wood, shelves that are either fixed with proper joinery or adjustable on sturdy pins, and a back panel that's actually attached, not just resting there. Run your hand along the edges in a store or look closely in photos online. Sanded and finished edges, square corners, no visible staples, that's the tell.
Weight capacity matters more than people admit when they're buying one. A shelf rated for 20 pounds is going to look sad and sloped under a full run of hardcovers within a few months.
Give it a flat wall, good light if you can manage it, and don't crowd it with too much else nearby. A bookcase that's allowed to just stand there and hold what it's supposed to hold tends to look right without any extra effort on your part.