The nose has practically become a vestigial organ in humans. Unlike other animals, who orient themselves to the world with their sense of smell, we rely primarily on our eyes, and then our ears. At least for Americans, there's a strange intimacy about personal smells. We wouldn't hesitate to say to an acquaintance,
You have a beautiful smile. Or,
You have sparkling blue eyes. We'd say,
You have a soothing voice, or
a great laugh.But would we say, You have a wonderful smell to someone we've just met? The oils in your scalp smell sweet and remind me of my father. Or, It's so warm in this conference room that I can smell your body really well. Sights and sounds are public. Smells are private.
This privacy is exactly why smells are so essential in fiction — their evocation in a scene provides immediate intimacy with a character.
Smells are also the most direct path to summoning up memories. There's a particular scent used in Japanese shampoos that I come across very occasionally in this country; when I do, I'm transported back to the two years I lived in Japan in the 1980s. 
A whiff of auto-body putty will take me back to a specific time of my childhood — age four — when we'd visit my grandfather in Vermont; his business sat next to a body shop, and although the putty smells sharp and toxic, it's pure comfort for me: the Green Mountains, a pond full of frogs, soft-serve ice cream, A&W root beer, Christmas and summer vacation, and steering the Pontiac from my grandfather's lap.
While writing my new novel, The Oregon Experiment, the power of smells to summon the past became especially clear to me. In the novel, all of the characters are either defined by their pasts, haunted by their pasts, trying to escape their pasts or reinvent them. Naomi has a genius nose, and when she's afflicted with anosmia — the loss of her sense of smell — she feels desperately cut off from her past (and also from everything around her since, unlike most people, she relies on her nose to orient herself to the world).
I don't have any sort of genius nose myself, but I'm interested in smells and pretty sensitive to them, so the research for this aspect of the novel was a joy. I read everything I could find on the sense of smell; Gabrielle Glaser's book The Nose: A Profile of Sex, Beauty, and Survival was especially helpful to me.
Since Naomi is a fragrance designer, I felt really lucky for the opportunity to spend a day with a San Francisco perfume designer, Yosh Han. Most big commercial perfumiers employ synthetic essences and chemists, but Yosh makes perfumes the traditional way — with 300–500 natural essences, and her nose.
On a sunny spring day in San Francisco I sat with Yosh in her studio and told her a little bit about my novel: Through a scientist at the university in Douglas, Oregon, who studies the inexplicable leaping power of the (fictional) Pacific leaping frog, Naomi gets a vial of a hormone secreted from a gland between the frog's legs that might be the source of its power. Perfume base notes are often unpleasant by themselves — musk, civet cat, ambergris (whale vomit that's been floating on the ocean surface for 10 years or so), and patchouli (which I, maybe in a minority, find pleasant on its own).
I told Yosh that I wanted the frog juice to evoke the duff in the Oregon Coast Range and auto body putty (yes, my own history weaves into the novel here), and she brought out her harshest smelling northwest essences — roots, mosses, mushrooms, barks. I picked the essences closest to what I had in mind, then we found the ones that worked well together. Yosh mixed them in ratios that experience told her would produce a good base note, and before long I had a small labeled vial: frog juice.
Next she brought out a few dozen more essences — middle notes and top notes. Cedar, fir, Willamette Valley mints, northwest grasses, daphne, lavender, and sage. In no time I was loopy, but she had me line up the vials that I liked, and sniff from base note to top, then back down again. We sorted, selected, edited, until finally I had something that evoked driving from the heat of Corvallis in the summer, up into the moist cool air of the coast range, and getting hit with that first whiff of Pacific sea salt. It's the fragrance that Naomi is trying to create in the novel — full of references to her past and the intoxicating experience of her present.
I wear the fragrance often, and since my own olfactory history is woven so tightly into Naomi's, and since the fragrance triggers the pleasure of my day with Yosh and the intense experience of writing the novel, I'm soothed, comforted, and stimulated by it, and I'm reminded of a full life.