A Natural History of the Senses, by Diane Ackerman ISBN-10: 0679735666
Not a how-to book for writers per se, but one every writer should take a look at. The book touches on each of the five senses, with an additional chapter for synesthesia.
Each chapter covers many topics, from Ackerman's personal memories to scientific studies to historical researches to philosophical asides. Did you ever think that tears are salty because we evolved from ocean creatures, and that our blood is salt water? Ackerman has.
But the book is more than just a collection of information; there's great writing here, and an aspiring writer could learn from some of these images and every page is filled with ideas.
More confessions here.