National Geographic
National Geographic: Encyclopedia: Mantle
An in-depth look at the structure and composition of Earth's mantle, with lots of visuals. Covers lithosphere, Mohorovicic discontinuity, asthenosphere, transition zone, lower mantle, the d double-prime region, mantle convection, mantle...
Science Struck
Science Struck: The Four Main Layers of the Earth
Gives short descriptions of the crust, mantle, outer core, and inner core.