There are several good sources for automobile valuation information.
(1) N.A.D.A. Official Used Car Guide. The National Automobile Dealer's Association's Guide is generally called "The Blue Book," even though it has a gold cover and even though there is another series of valuation books called "Blue Books" (discussed below). The Guide tells you the average price dealers paid for trade-ins. It is widely used by dealers, insurance companies and people valuing estates for tax purposes because its figures are low.
There are several different editions of The Blue Book for different regions of the country, and NADA publishes other Guides for other kinds of automobiles (motorcycles, trailers, "classic" cars, etc.). The N.A.D.A. Web site list all the Guides, but somewhat obviates the need for most of them by letting you look up the value of most kinds of vehicles sold in the last ten years. You can find out the price for older cars by calling N.A.D.A. (800-966-6232; $30 per request in 2011) or getting copies from N.A.D.A.'s Older Used Car Guide, which is available in many larger public libraries.
(2) Price Digests The Price Digests series includes The Automobile Red Book, known as "The Red Book," as well as a number of "Blue Books" covering other kinds of vehicles (e.g., the Truck Blue Book, the Horse Trailer Blue Book, the ABOS Marine Blue Book, etc.). There are special Blue Books for older vehicles (e.g., the Older Car Blue Book, the Older Truck Blue Book), boats and airplanes. These are available in print or by subscription through PriceDigests.com.
The publisher's stated goal is to be impartial and accurate. That is, the price in The Red Book should be a useful measure of what a car is really worth on the open market.
Note: Between November 15, 1999 and 2002 The Automobile Red Book was titled the NMR Auto Blue Book. Also, at various times, the publisher has called itself "National Market Reports," "Mclean Hunter Market Reports" and "Blue Book Values" before being purchased by Primedia and renamed "Primedia Price Digests." When I checked again in 2008, the publisher was Penton Media, Inc.
(3) The Kelly Blue Book Price Manual. This is also put out by a private company. There are different versions for new and used cars; there is also a user-friendly Internet site (www.kbb.com). The Manual's values are based on average prices paid at recent car auctions (usually by dealers) plus transportation, cleaning and repairs. Historical valuation reports can be purchased by calling 800-258-3266, option 2. Note: The "Kelley Blue Book" should not be confused with "The Blue Book" by NADA or the "Automobile Blue Book."
(4) Black Book. Black Book produces several print and online valuation guides based largely on auction sales. They cover cars, light trucks, snowmobiles, ATVs and other vehicles (boats, etc.).
(5) Other electronic sources. The consumer-oriented Edmund.com provides a wealth of information about buying new and used cars, including "prices, ratings, reviews, specifications, recalls, etc," as well as safety and insurance information. TrueCar.com sells New Car Reports will tell you the dealer's real cost for a particular model (the invoice price, the "holdback" discount, special incentives, etc.) plus actual local sales data. For a modest fee, CarBargains will try to get the best new car price on any make and model you request; Authority Auto is an upscale higher-priced car shopper site.
For older car prices you can also check Classic Cars, Mecum
Auctions and Collector Car Market Review, as well as the relevant sections of the NADA, Kelly and Edmunds sites. CarGurus locates used cars currently listed for sale in a particular zip code.
Damage, Fraud: For a small fee, Carfax will check a used car's identification number to see if it has ever been involved in a reported fraud situation (e.g., an odometer roll back), if the title is encumbered and to see if has ever been wrecked and retitled, and to see whether there have been manufacturer recalls. Carfax Vehicle History Reports are also available through Accurint. Experian sells an equivalent report called an AutoCheck Vehicle History Report.