There is no comprehensive, detailed directory for CPAs, but there are a number of free nation-wide CPA look-ups, such as CPADirectory.com and the links in ABC Search Engine. If that doesn't work, you can see if the accountant is licensed in a state that posts an online lookup; links are posted by BRB Publications and SearchSystems.net. Alternatively, you can use the fee-based professional licensing databases on Lexis (ex. NJ has NJ;NJLIC), Accurint or another vendor. For other states, call the relevant Board of Accountancy/Licensing (e.g., 518-474-3836 in NY, 973-504-6380 in NJ).
Alternatively, you might try calling the American Institute of Certified Public Accountants (888-777-7077) to see if the person in question is a member. (New York Note: Calling the AICPA for New York checks all CPAs who "registered" since 1980 -- which does not include some CPAs who originally registered before 1980 and either work in private industry or out of state. To check for EVERY CPA in New York, you must send a written request with a $10 check to the New York State Education Department / Records Access Unit / Division of Professional Licensing Services / Cultural Education Center / Albany, NY 12230.)
To find professional publications by a CPA, search the databases discussed in the "Accounting" entry.
Ethics: The Professional Ethics Executive Committee (PEEC) of the AICPA publishes and posts a Code of Professional Conduct with Guide for Complying With Rules 102-505.
To find out if a CPA has been disciplined, contact the relevant state licensing board; links are posted by the AICPA.