I've had a small amount of success with D-ribose too, but not in the superlative way the book leads you to believe. It's just a slight energy boost within the first hour or two after taking the twice daily dose. But I still find it worthwhile, so I continue to take it.
Inester, it gave me stomach problems too at first, but I think that was because it wasn't dissolving completely in the water. After a while, I started mixing it in very hot water (2.5 minutes in a microwave for a 12 oz mug), which ensured that the Ribose fully dissolved, and the stomach issues dissipated.