This is a thorough overview to Edgar Rice Burrough's (creator of Tarzan and John Carter) 7 book series. For those of you unfamiliar with this tale, it chronicles the adventures of David Innes and Abner Perry who travel to the center of the hollow earth, where prehistoric creatures and cavemen still exist. Critchfield supplies maps, diagrams, articles, belief systems and a glossary of names/creatures of Pellucidar.
It's a thorough book but it didn't tell me anything I didn't already know (I've read the series several times). However, it's a good introduction for someone who has just discovered these novels.