It ain’t elegant but more than ten years later it’s still holding up my books.
I’m not going to do a step-by-step for this one. Your circumstances are likely to be different, and I think a brief description along with the photos will explain the theory well enough. (Not to mention that to do a proper step-by-step I’d have to take all the books down, and that’s a good deal more work than building the shelves!)
The 5 foot boards are supported in a cradle of loops of baling twine—three loops for shelves meant mainly for paperbacks, and four for hardbacks and larger books. Each loop requires two screw eyes, one attached to the bottom of the rafter and the other the width of the board away to the underside of the loft itself. Getting the board to sit (reasonably) level is a bit finicky and will probably require some untying and retying knots until you are satisfied. At this point the shelf will still look a bit iffy. Never fear! Start adding books and watch it stabilize.