Mundus Subterraneus

Mundus Subterraneus is an interesting site that features:

A bibliography of literature on the Hollow Earth, subterranean worlds, worlds beyond the poles, the Secret World, the centre of the Earth, the earth's interior, the hollow globe, Symzonia, Geo-Kosmos and the cellular kosmology.

See also Arktos by Joscelyn Godwin on these topics.