What should I consider when designing a simple bookcase that will be built without power tools?
The solution when I was in college was to use cinder blocks as the uprights. No tools at all required. This is a long time ago granted, but I believe gravity has not changed much. Cheap and common, cinder blocks are about 8 inches square by 16 long. Two stacked for each end of a shelf makes room for common book sizes. We would call 22 mm 3/4inch here and I would comfortably make a four foot shelf for books without worrying about bracing provided it is solid wood, which it sounds like it is. The advantage of the cinder block approach is ready availability and quick installation. It is not pretty, though, you would have to call it utilitarian, which went well with the wire spool table and orange crate record storage. This is, or was, a common approach to furnishing your first apartment in the States. They do make cinder blocks in 8 inch cubes also if your shelves are narrower.