Perplexed consumers will find that help, in many cases, is as near as the corner mailbox. Montgomery County's Office of Consumer Affairs has deciphered many of the letter-and-number codes stamped on food products which often reveal when the products were made and what their shelf life is. Crack the Codes is free for county residentsts (25 cents for non-residents) from the Office of Consumer Affairs, 611 Rockville Pike, Room 201, Rockville, Md. 20852 (include a self-addressed envelope with 20 cents postage).

A brochure from the Civil Aeronautics Board should trigger a surge of assertiveness in airline passengers. Fly-Rights gives tips on how to find the lowest fares, explains passengers' rights when luggage is lost or flights are overbooked, and offers advice on lodging complaints. It's free from the Consumer Information Center, Pueblo, Colo. 81009.

The League Women Votes' pamphlet What's What in Metropolitan Washington guides citizens through the structures and functions of the area's local governments. It costs $1 from 10408 Johnson Dr., Manassas, Va. 22110.

And, after bolstering your confidence with these guides, arm yourself for a full-fledged investigation with Raising Hell, a pamphlet by Dan Noyes of the Center for Investigative reporting which explains how to use public records and libraries to investigate individuals, corporations and government. It's $1.20 from Mother Jones, 625 Third st., San Francisco, Calif. 94107.