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Two books which describe the operation of the digestive
tract are a good starting point for one to get an
"Owner's manual" like view of how things work.
The first is "The Inside Tract" by Myron D. Goldberg,M.D.
and Julie Rubin distributed by AARP.
This is a highly readable book which touches all the
bases and has a particularly good section on the
function of vitamins and minerals in the body. It
doesn't deal with celiac disease per se but it covers
many of the medical conditions in great detail.
The second is "Indigestion" by Henry D. Janowitz, M.D.
This book in available from Consumer Reports magazine.
It does treat celiac disease although that isn't the
major topic. The author brings his insights
from many years of practice to the lay reader.
I think both books would serve to give one a good
overview of the most common GI problems along with their
causes and cures.
Readers of this group range from those with diagnosed CD
to those who have a problem and just suspect CD. In between
are those with certain food intolerances or other problems
that may or may not be CD. So a bit of education is in
order.
 
There is a marvelous book for those who are curious
about the physics and chemistry of food. It is "On
Food and Cooking" by Harold McGee 683 pages
Collier Books copyright 1984 available in paperback.
Chapter 6 is "Bread, Doughs, and Batter"
On page 284:
"Three basic kinds of wheat are grown today...
The distinguishing factor is kernal "hardness"
which is a measure of protein content... Hard
wheat grains break up into large chunks of
protein and form a strong gluten when the flour
is mixed with water; hard wheats are therefore
preferred for bread making and constitute about
75% of the American crop. Durum is the hardest
kind of wheat grown; too hard, in fact, for
bread dough, which must have some give to it.
It is usually milled into a coarse product
called semolina, which is used to make the very
stiff doughs necessary for dried pastas."
On page 285:
" "All-purpose" or "household" flour is a blend
of hard and soft flours meant for use in a wide
range of foods."
There is also a table which breaks down the protein
content as follows:
Whole wheat     13.3%
Semolina        12.3%
Straight hard   11.9%
All-purpose     10.5%
Straight soft    9.7%
Cake             7.5%
 
I find this book to be fascinating.