Martindale-Hubbell is the leading directory of attorneys in U.S. law firm. The set also has listings for some foreign attorneys, law schools/professors, state bar associations, plus summaries of the laws of all states & many foreign countries ("Law Digests"), the Model Code of Professional Responsibility (with a cross-reference table between the Code and the Rules), model codes, selected Hague conventions and a few other treaties. Also, Martindale-Hubbell provides a quality rating for many attorneys. And after the name of each city, town and village in the blue pages you'll see the population and the name of the county it's in. Only the paper edition has all the features.
You can look up most lawyer and law firm entries for free on the Martindale Web site.
For sophisticated searching, the lawyer directories and most other parts of the set (except the treaties) are available on Lexis.
Old Editions: Some large law libraries keep a few years worth of old Martindale-Hubbell sets, and a few keep them way back. Many large law libraries keep old editions of the volume for their state. These libraries are your best sources for getting copies of historical entries.
For example, you can get copies from old New York City volumes from the New York County Lawyers Association Library (212-267-6646, x205 or x206), and you can get copies from old Ohio volumes from the Cleveland Law Library Association (216-861-5070). For one-stop-shopping, I believe the Columbia Law School Library has all the old Martindale-Hubbell sets, either on microfiche or in hard copy; you can get copies from their document delivery service (212-854-5445 for library members; otherwise, call 800-332-4529; for information, see http://web.law.columbia.edu/library/services/fee-based).
Martindale-Hubbell Law Digests: The Martindale-Hubbell set used to include "Law Digests" by leading law firms that summarized the laws of the U.S. states, the U.S. Federal Government and many foreign countries. Until 2007, the Law Digests were published annually as two print volumes accompanying the print Martindale-Hubbell Lawyers Directory. From 2008-2010, the Law Digests were published annually on CD-ROM. In 2010, Lexis announced that they would not update the U.S. Law Digests in future years. The Foreign Law Digest were expected in 2011 but not published again.
Last I checked, the 2010 U.S. Law Digests were still available on Lexis (e.g. MARHUB;alDIR for Alabama) the last time I checked, and you could still buy downloads of the 2007 Digests from Amazon.com.