Here's a great documentary on Kirby's life and work I just watched:

Jack Kirby: Story Teller (Jack Kirby art) Full documentary


Comics creators and friends shown include Len Wein, Marv Wolfman, Mark Evanier, Steve Sherman, Walt Simonson, Steve Rude, John Romita Sr., two of Kirby's own children who lived with him across those decades, along with many other comics writers and artists.
They discuss Kirby's art style and influence, their personal relationships with Kirby, his personal life and work habits, and the major events of his life as a comics professional.

One part that was really interesting is that what Kirby wanted but was denied at both Marvel and DC in the 1960's and 1970's, as far as royalties, creator ownership and profit sharing, have been part of Marvel and DC contracts for any starting artist since 1985.
Just after Kirby's career ended. That if Marvel had just given him that kind of a contract in 1969, he probably would have spent the remainder of his career at Marvel.