I'll have to check out that Curtis book - thanks, GourmandHomme! Looks like a very good one.
Another wonderful book just came out - it provides an introduction to perfumery, including the use of both naturals and synthetics. The Essence of Perfume
by Roja Dove, ISBN 978-1-906155-49-0
Also expensive, but worth the money. It's a beautiful and conversation-inspiring coffee table book, so maybe somebody can be talked into paying for it.
Covers all the major natural and synthetic ingredients of perfumes, but in a high-level way - not like the tremendous detail you will find elsewhere.
I'm a chemist, so I'll try to give you a brief answer to put things in perspective. Natural scents like a rose contain specific mixtures of natural organic substances. The fragrance you smell from a real rose can be mimicked rather well by a judicious combination of those same chemicals, which may come from (1) real roses, or (2) other, cheaper, natural sources, (3) by chemical conversion of cheaper chemicals from other natural sources, or (4) from purely synthetic organic chemistry. Remember - it's almost all "organic". The molecules are the same molecules (at this stage of my answer). The question is where those natural molecules came from - "natural", "semi-synthetic", or "synthetic" means. But chemists and perfumers have discovered that slight variations on the molecules that occur in nature (whatever their source) give rise to molecules that - while tending to have many of the same structural features as natural molecules - are completely new. These new molecules are often highly prized, since they not only can duplicate smells in nature in a more simple fashion - they can even offer a scent unlike anything smelled before. "Better than nature", in a way. It is by combining both natural and "new" synthetic substances that the great perfumes are born. The science and art of this combination is perfumery
Thus, to make a rose scent, you don't need all the components of the odor of a rose. You only need enough to make people think it's a rose. And if you deviate from the recipe in the correct ways, people will say that what you made is like the best rose, or even better than a rose.
Roja Dove's book really illuminates the importance of synthetics in the history of perfumery, and using very specific examples. I can't recommend it enough. Good luck with your science project!