Books:
Out of Poverty by Paul Polak
Design for the Real World by Victor Papanek
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Articles:
“Design Thinking for Social Innovation” by Jocelyn Wyatt and Tim Brown of IDEO (Stanford Social Innovation Review)
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Papers, Essays & Guides:
The Human Centered Design (HCD) Toolkit by IDEO, IDE & The Gates Foundation (free download – PDF)
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If you have more to add to this list, please leave them below in a comment. Thanks!