That's a beautiful copy of DD Beach Party. In terms of keys, you could say that almost every appearance in Four Color is a key Duck book, because they're all classic stories. But using the normal definition of key, my list off the top of my head would be:
FC 9 (the uber-key, the AF 15, Showcase 4 and Action 1 of duck-dom: the first Barks duck book)
FC 178 (first Uncle Scrooge)
FC 199 (classic cover)
FC 386 (considered to be first issue of Scrooge in own title)
US ? (first Magica deSpell)
US 64 (treasure of Marco Polo, banned by Disney from being reprinted for many years)
WDC&S 98 (first Uncle Scrooge appearance in WDC&S)
The first FC with a Scrooge cover, and the first Flintheart Goldglom appearance, would also be keys, but I'm spacing on which issues they were and I don't have a copy of the Guide at work with me. Arty and hkp are lightyears beyond me in terms of Duck expertise, so perhaps they could contribute to this list.
To keep this thread going, here's a minor key, WDC&S 140, first appearance of everyone's favorite egghead, Gyro Gearloose.