In Worlds Without End: The Many Lives of the Multiverse, Rubenstein traces theories of multiple worlds from ancient philosophy to modern physics. I found the text thought-provoking; as someone with a background in philosophy, I found it highly readable though admittedly, some parts were easier to follow for me than others (ie. I very much appreciated Nietszche's appearance toward the end but don't know that I can say I really grasp the cosmic microwave background). Perhaps most important, the book left me with more questions to ponder than I had when I started - for this, I'm appreciative.