Summary:
Doll guides are large books for doll collectors
Doll guides are large books for doll collectors, used to help them tell one doll from another. They generally have large colour photos of each doll, together with price information. Thanks to the popularity of doll collecting, doll guides sell surprisingly well, even though they are often large, thick, expensive books. They are generally used mainly by professional collectors as tools for identification and valuation.
Because it would be pretty much impossible to fit every doll ever made into one book or even a series of books, each book tends to concentrate on a particular decade, or manufacturer, or even one doll (Barbie, for example).
Doll guides can be quite hard to find in shops (unless you live near a specialist doll shop), but they're easy to find on the web, particularly on Amazon.com and eBay. If you get one used, it might not even be all that expensive, although you should expect to have to pay a lot for postage. Make sure you read reviews of any given book before you buy it, because they vary hugely in quality, and not all of them do what they say on the cover very well at all.
It is ironic that doll guides are easiest to buy on the web, however, because it is also the web that is making them slowly obsolete. Even the best doll guides can't hope to even touch the sheer diversity of pictures and information that exists on the web in the doll collecting enthusiast community. There are websites out there that are at least as reliable as any of the books, and websites are usually more up-to-date in terms of price guidance.
The web works particularly well if you know which doll you have and just want to know which type it is. Even if you know nothing about the doll, however, and you can't find it in a database, it's not hard to find doll collectors' forums (search for 'doll forum'). Once you're on a forum, you can post pictures and other enthusiasts will help you out - someone is bound to know the answer.