I have found a great book about Vitamins and Minerals called "A Doctor's Guide to Vitamins and Minerals" by Mary Dan Eades, M.D. I think the title is a little misleading because I find it is really easy to understand and appears to be written for the layman's point of view. It's about the therapeutic uses of vitamins, nutrients, and minerals -- alone or in combination.
Here are just a few of the things the doctor mentions about Calcium.
1. Among foods that interfere with the absorbtion of calcium are cocoa, soy beans, and foods with a high phosphate count (carbonated beverages.)
2. Protein in the diet influences calcium absorbtion as well. On a high protein diet about 15% of the calcium taken in by mouth is absorbed, while on a low protein diet only 5% is absorbed.
3. Caffeine increases the loss of calcium through the kidneys.
4. Taking daily doses above 2000 mgs of calcium a day can cause hyperparathyroidism.
There is more in her book. The major book ordering websites have it.

Hope this helps someone -- Steve