Given that over 40 years translates out into a lot of comics, and given that the official collectors edition dvds and Marvel Digital Unlimited mean that almost all of this backlog is easily available, it's sometimes hard to find the kind of issue you're looking for. Namely, one in which the characters you're interested in actually interact.
To that end, I've tried to create a very simple, extremely informal guide to Avengers comics written before Marvel's Civil War.
Note: In no way are most of these intended as in depth reviews or judgement calls. They're just meant as a way of telling how much relevant Iron Man and Captain America content an issue has and whether the issue would be of significant fan interest - with slight snark thrown in to keep me from getting bored.
NOTE: At this time, this project is incomplete. Most of Avengers vol 1 remains unaddressed for the present. More forthcoming eventually.
GUIDES TO THE AVENGERS:
The Avengers vol 1: The 90's
The Complete Avengers vol 2
The Complete Avengers vol 3
The New Avengers, issues 1-20, up to Civil War