Falling between the cracks are oodles of modern buildings that are not old enough to qualify for the National Register of Historic Places (+50 yrs old). Like this Usonion House designed by Frank Lloyd Wright, for instance.
That's where the Recent Past Preservation Network comes in. They are dedicated to documenting and providing education on modern buildings that have made a contribution to design and are now at risk.
Their website is really a wealth of information on modern building design nationwide, and it's easy to find yourself completely emersed. Be sure to check out the National Windshield Survey, which documents endangered and already-demolished structures from coast to coast.
While many of our NYC structures are protected by the Landmarks Preservation Commission, the building stock in a city like LA is made up of many unsung modern masterpieces that have no means of preservation in place.