The Number Of The Beast by Robert A. Heinlein

A rollicking, genre-blending adventure in which a small group of friends and family use a customized craft to hop between alternate universes, slipping into worlds drawn from myth and literature as well as stranger, parallel realities; amid encounters with familiar fictional characters and bizarre new societies they face external threats and internal strains while wrestling with questions of identity, free will, and what it means to belong in a cosmos where fiction and reality blur.

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