Amazing Spider-Man is the best place to start, I'm not savvy enough with his canon to know what issues would be truly essential but this list (http://www.comicbookdb.com/character_chron.php?ID=444) is not terribly long. This (http://marvel.wikia.com/MacDonald_Gargan_%28Earth-616%29) has helpful issue footnotes as well! I'm sorry I can't be more helpful. But I would love to see a Mac Gargan around.