Most of the value of first editions lies in the condition. And virtually all of the collectors of first editions collect "modern firsts", that is, books published since the novel became popular in the 19th century.
Collectors of first editions tend to collect fiction. Not universally, for sure, but a very large portion of the market is in fiction. The most important works of nonfiction will be collected and appreciate in value, and generally, they are worth something, but not the sky high prices seen for some modern firsts.
The biggest factor in the value of first editions is the condition of the dust jacket - a dust jacket in extremely good condition may easily be worth three times the book that it is on. The dust jacket should, however, be the proper dust jacket for the printing of the book - small changes are often made that distinguish a first printing dust jacket from a later printing of the first edition, and sometimes, there are even variations within the first printing dust jacket. Also, as with sports cards, there is massive difference between the prices of the cards in very good and near mint conditions.
For example, on bookfinder.com, a first edition, first printing of The Grapes of Wrath by John Steinbeck in "fine" condition with a "fine" dust jacket lists for $12,500, while a copy from the same seller, with slight darkening to the spine of the book, and two minor tears to the dust jacket, repaired, and described otherwise "fine" lists for $6500. A copy in "very good" condition with a " very good" dust jacket lists for $1400. Without a dust jacket, a copy in "good" condition lists for $300, and on eBay a copy of the first printing, in "fair" condition, with a later dust jacket, recently sold for $122.50. This is for a text that can be purchased new, with the same information that is in the first edition, in virtually any bookstore, for a small fraction of the price.
To navigate through this, a very good source is Collected Books : The Guide to Values, by Allen and Patrica Ahern (New York : G.P. Putnam's Sons, 2001). This book, in addition to just listing values, lists the points, that is, the elements that allow one to distinguish between a first printing and later printings, and between first printing and later dust jackets.
Yes, this is a bit ridiculous. But isn't most collecting?