My preceding post got rather technical and delved into the finer points of cell biochemistry. For those who want to learn more about how cells work, I thoroughly recommend Metabolism at a Glance (Paperback) by Jack G Salway, Sen. Lecturer in Medical Biochemistry, University of Surrey. It's crammed with metabolic pathway diagrams.
I also recommend Medical Biochemistry at a Glance (Paperback) by Jack G Salway.
For general information on nutrition and metabolism, I recommend Introduction to Nutrition and Metabolism (Paperback) by David A Bender, Sen. Lecturer in Biochemistry, UCL.
A good website for information on how the body works is http://www.medbio.info/.
The best site for information on fat loss, muscle gain etc is http://www.bodyrecomposition.com/.
A good website for enzyme structures is http://www.ebi.ac.uk/thornton-srv/databases/enzymes/.
Here's a searchable version of Biochemistry, by L Stryer.
It's a bit dry, but of interest is:-
Food Intake and Starvation Induce Metabolic Changes and
Phosphatidate Is a Common Intermediate in the Synthesis of Phospholipids and Triacylglycerols.
Here's NCBI Bookshelf where you can search many textbooks.
Update: Here are some cool Animations and Computer-Generated Movies by Harvard University.
Here's a YouTube video of ATP Synthase, which takes a proton gradient and, using a molecular motor-generator, converts ADP + Phosphate into ATP, the energy source that cells use.
Happy reading and viewing!