For Us, the Living: A Comedy of Customs

Considering that this is an early work, it's quite good. There are clear references to characters who appear in his later works - Lazarus (of course!), Professor Bernardo de la Paz - as well as themes, most of which show up here to some degree. While I liked the book overall, it did feel less polished than many of his later works. If you are looking to try Heinlein, I would not recommend this book (pick something like The Moon is a Harsh Mistress, or Friday) but if you are an established fan, I think you'll get something out of this one.