The Aircraft Electronics Association (AEA) has published a manual, the "Required Avionics Handbook," that many operators and avionics shops will doubtless see as a godsend.
The handbook is expressly designed to help operators of both Part 91 and Part 135 aircraft figure out what avionics they need in their airplanes. With tabs showing the required avionics for both small (FAR Part 23) and air transport (FAR Part 25) aircraft, the handbook essentially indexes the requirements that the FARs reference in four different sections over hundreds of pages.
AEA planned to have the handbook available online (www.aea.net) by April 1st. Changes, corrections and updates to the publication will be posted on the site, as well. Non-AEA members can purchase a copy of the handbook for $50.