There are a number of conservation laws in physics, not just the conservation of energy, that you will encounter throughout this book: (linear) momentum; angular momentum; and electric charge. The total number of electric charges, for example, is constant in time; charge cannot be created or destroyed in a closed system.
When doing physics at the atomic level, there are two other conservation laws governing elementary particles that are important: the number of baryons (such as neutrons and protons); and the number of leptons (such as the electron, muon, and neutrino). But don't commit that last one to memory yet: recent scientific results on neutrino oscillations appear to violate the lepton number conservation law and may point the way to exciting new physics!
Most of the time we can also think of mass as being conserved. But Albert Einstein put forward a model using his famous equation, E=mc2, in which mass-energy—not mass—is conserved. In Einstein's equation, mass can be converted into energy and vice-versa, so you cannot consider the total mass of a closed system without considering the total energy, too. Einstein's equation becomes important in atomic physics and for physics at very high speeds (approaching the speed of light). 
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