Getting Started with Arduino Book (paperback) (3rd Edition)
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If you've heard about this Arduino thing, but are unsure about where to begin, Getting Started with Arduino is a great book to begin with. 245 pages of easy to read, well written, Make/O'Reilly goodness. Massimo Banzi (author and one of the 5 Arduino developers) describes the basic hardware, writes some simple programs, and explains how to use the free Arduino development environment.