Billy bookcases are great if you use them right. Best to get the narrow models and fix them next to each other with carefully placed wood screws.

This makes super rigid and durable shelves for heavy books and the shelves never noticeably sag because the span is so short. You can also customize the heights of the shelves with fine granularity.

The only other bookshelf system I use is the "Hungarian Bookshelf" (https://www.instructables.com/id/Hungarian-Shelves/). You basically make this yourself with wood. Excellent for the basement.