In a former life, I hosted cycling events. I recently found a stash of leftover bib numbers from the early days of hosting the Hopkins Park Cyclocross Race in the Chicago CX Cup series. So, I decided they'd be better suited as book covers than in the basura. Tyvek is actually an excellent material for pocket books - waterproof, tear-resitant and bleedproof.
The bib numbers are mounted to 100lb French Paper card stock and filled with ~60 pages of 60lb paper.