Where can FAA Letters of Interpretation be found?
The FAA Regulations Division maintains a webpage which includes a searchable database of "Legal Interpretations &amp; Chief Counsel's Opinions".  It includes the following disclaimer:


  Legal Interpretations and the Chief Counsel's opinions are now
  available at this site.  Please note that not all interpretations or
  Chief Counsel's opinions are available at this time.  This database
  consists of legal interpretations issued from 1990 to the present and
  will be updated on a regular basis.


The letters can be searched by year and/or keyword.