The plan when tasting Zinfandel is to taste five iterations of the grape, to get a sense of what it tastes like, and whether you like it. To do that you'll contrast both top shelf and bottom shelf; you'll contrast terroir-driven, or site-specific single-vineyard wines with one another; you'll taste the most critically lauded version of the wine; and you'll taste a late-harvest to learn what the grape tastes like fully ripe.
The hardest part of learning about wine is shopping for it. See the full field guide [in the book] for detail. This brief list is from it. Don't worry about the exact names mentioned here, try to match the category, not the specific wine. For your tasting you want the following:
1) Inexpensive American Zinfandel, such as Rosenblum, Rancho Zabaco (or Dancing Bull), Ravenswood, Cline, Beaulieu Vineyards (BV), Montevina, Seghesio, Gnarly Head, and Renwood. This should give you a basic, average taste of what Zinfandel tastes like -- think of it like a basic tub of vanilla ice-cream, not a very fancy chef-made single scoop.
2 & 3) Two different single-vineyard bottles from different vineyards but made by the same producer. You could buy Ridge's Lytton Springs and Geyserville, for example. Or Edmeades' Piffero Vineyard and Perli Vineyard, Rosenblum's Rockpile Road and Lyons, Bella's Big River Ranch and Lilly Hill, or pair from Green and Red, Benessere, Robert Biale Vineyards, Carlisle, Dry Creek Vineyards, Franus, Peterson Winery, Ravenswood, Seghesio, Joseph Swan Vineyards, Renwood, and Bucklin Old Hill. Basically, you want one producer but two different wines from two different places, to tell you about how place affects taste.
4) A more expensive, polished Zinfandel, such as Gallo-Sonoma, Turley Wine Cellars, Dashe Cellars, Beaulieu Vineyards (BV), Behrens and Hitchcock, Fife, Hartford, Howell Mountain Vineyards, Martinelli, Murphy-Goode, Peachy Canyon, or Storybook Mountain Vineyards. Ask the guy at the wine store for the roundest, ripest, richest, fullest, lushest Zinfandel he's got. For this you're looking to taste how the most expensive wine making known to man affects taste.
5) A late harvest, ice-wine, dessert-wine, or Zinfandel "Port." These often come in half-bottles, and that is enough for a dinner-party-sized group; any late-harvest Zinfandel will do.
Excerpted from Drink This: Wine Made Simple -- Everything You've Ever Wanted to Know About the Most Wonderful Drink on Earth by Dara Moskowitz Grumdahl. Excerpted by permission of Ballantine Books, a division of Random House, Inc. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.