Alton Brown's cookbooks explain recipes in a scientific way. As a matter of fact he was a scientist before he was a cook.
http://www.altonbrown.com/adventure/books.html

I second the Alton Brown books! My cousin is a fan of Cooks Illustrated but I'm not sure how science-y the magazine is.

My all-time favorite cookbook is "The New Best Recipe Cookbook" (by the makers of Cook's Illustrated Magazine) -http://www.amazon.com/The-New-Best-Recipe-All-New/dp/0936184744/ref=sr_1_1?ie=UTF8&qid=1356360891&sr=8-1&keywords=new+best+recipe+cookbook. It includes TONS of explanations as to why they developed each recipe the way they did - it's really fascinating and I have learned so much about cooking & baking from them.

Check out Michael Ruhlman's books. Most of them break down cooking in a scientific way.

If you want a great book that explains a lot of interesting details about cooking and the science behind it, purchase "On Food and Cooking: The Science and Lore of the Kitchen" by Harold McGee. This is a very popular book among chefs.

I'm surprised no one went right to "The Science of Good Cooking" by Cook's Illustrated. http://amzn.to/UXkxRw I don't own it (yet) but it seems to be exactly what Mary is looking for. Maybe someone who has a copy can comment? Also, I definitely second Joe's recommendation of "On Food and Cooking" by Harold McGee. That's one I DO own and love for it's science-first approach to food.

Actually, I believe he was a drama major in college and picked up the science when he was training to be a chef.

The Science of Good Cooking is organized by concepts but includes many delicious recipes with detailed scientific explanations. I love this book!
http://www.amazon.com/Science-Cooking-Cooks-Illustrated-Cookbooks/dp/1933615982

The first book that got me interested was - What Einstein Told his cook - http://www.amazon.com/What-Einstein-Told-His-Cook/dp/0393011836/ref=sr_1_1?s=books&ie=UTF8&qid=1356371324&sr=1-1&keywords=what+einstein+told+his+cook
McGee's "On food and cooking" is awesome, but very wordy and not necessarily an easy read (although I have read it twice)

Ruhlman's books (Twenty, Ratio, Charcuterie, new Salumi, among others) are all really good at explaining techniques and procedures.

"Cooking for Geeks" is also really good - http://www.amazon.com/Cooking-Geeks-Science-Great-Hacks/dp/0596805888/ref=sr_1_1?s=books&ie=UTF8&qid=1356371465&sr=1-1&keywords=cooking+for+geeks

Haven't picked up "The science of Good Cooking" But I'm sure anything from America's Test kitchen will be awesome

I second nossi1023's reccomendation for "Cooking for Geeks." It's not really a cookbook, but it's a great scientific supplement to cooking that you can apply to recipes you already have.

I second _What Einstein Told His Cook_ as a great foundational text on this topic!

I would also recommend The Science of Good Cooking. I've only checked it out from the library, but I want to buy a copy because I like it so much! It gives a lot of good detail about the science of each of the techniques they feature, and for the most part it seems pretty solid (I'm a chemistry PhD student). I've loved reading through it!

Any Cook's Illustrated. Maybe not the most "scientific" if looking at it from a Molecular Gas level or an Alton Brown...but many of theirs have long descriptions of why they tested certain things, why they want you to use certain products, etc. They consistently make me feel like I have more knowledge when I cook and not just a recipe.

Also the updated McGee book. His stuff is great and there's a reason it's mandatory reading at all the cooking schools.

I just gifted The Science of Good Cooking. One went to my boyfriend, the other to his parents. He opened it and proceeded to shout fact to me for the next two hours. I would say it's a great book, even though I haven't had a chance myself to look through it!!!

I second Harold McGee - On Food and Cooking.
I use it regularly, I also have an Alton Brown book, but On Food and Cooking is better by far.