I find Mark's Daily Apple really helpful. Check out this about grains:
http://www.marksdailyapple.com/definitive-guide-grains/
Paleo eating sites have also really impacted my choice to go grain free.
http://paleodiet.com/ Paleo nutrition/diet folks have the strongest argument (IMO) on why NOT to eat grains. Although reading stuff about the SCD and GAPS diets is what inspired me to go grain free in the first place. (_Gut and Psychology Syndrome_ by Natasha Campbell McBride was a great read IMO.)
Sorry, these are not all book recommendations. My favorite cookbook, which has a strong argument for going grain free (along w/ going pretty paleo as well) is
_The Garden of Eating_. They discuss various hunter gatherer cultures and their food intake, their rates of cavities and diseases (taking lots of info from Weston Price's NaPD) and I think have a great argument for why grain free is a healthier way to be...
I mostly buy cookbooks personally, but enjoy _The Primal Blueprint_ (although I think you can get most of the info from the book on the
Mark's Daily Apple site), and am longing to get my hands on Neanderthin. But I haven't bought/read _The Paleo Diet_ book, and Cordain's anti-fat stance is the reason. Although I believe it might have very compelling info on why to drop grains...
HTH!!