Have you ever wanted to throw yourself into a fantasy world? Tour through Middle Earth? Plan a trip in Narnia? Who hasn't imagined themselves rubbing elbows with Robin Hood or Jason's Argonauts?
Some folks have gone so far as to write their own cross-world adventures , much to the delight of their readers. L. Frank Baum made it a common practice to feature immigrants from the "real world" to Oz. L. Sprague de Camp and Fletcher Pratt, in their Incomplete Enchanter, detailed the travels of Earth-dweller Harold Shea through Norse Mythology and The Faerie Queen.
And now, the esteemed Poul Anderson has taken a stab at the genre with Three Hearts and Three Lions.
(see the rest at Galactic Journey!)