The under-the-radar books early retirees and savvy investors are reading to get ahead

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• Business Insider rounded up the books that investors say have shaped their thinking about money.
• One financially independent NYC-based couple drew inspiration from "Die With Zero."
• Another savvy investor thinks "Work Optional" is an "incredibly underrated" money book.

If you've read a business or finance book roundup, chances are "Rich Dad Poor Dad" made the list.

So many investors have cited Robert Kiyosaki's...