<i>Medieval Technology and Social Change.</i>  The title sounds boring, but this one of those books that just cranks your brain open.  There is so much new stuff in it.  Or at least, stuff that was new when it was published in the 1960s.<p>Carlo Cipolla's <i>Guns, Sails, and Empires</i> is a close second.  His <i>Clocks and Culture</i> is also very good; the prologue is probably the best 21 page summary of the history of western technology that you could find anywhere.