This is actually probably the best time ever to collect Kirby. Much of his work prior to the 1960's wasn't very accessible, until the last decade, due to how few copies there are out there, from the 1965-and-prior comics era.

But as Profh0011 pointed out, much of Kirby's early work is now being reprinted, and extensive checklists of Kirby's work are also making it easier to seek out Kirby's massive body of work.

I highly recommend THE ART OF JACK KIRBY (by Ray Wyman and Catherine Hohlfeld, 1992, The Blue Rose Press). Which in addition to covering Kirby's entire career, has an extremely detailed checklist and chronology of Kirby's work. A tremendously useful reference tool.

Two other great volumes are the collected hardcovers of BOYS RANCH and FIGHTING AMERICAN.

And of course, the Marvel Masterworks collections of virtually all Kirby's early Marvel work on FANTASTIC FOUR, THOR, AVENGERS, and X-MEN.

All in beautifully reprinted hardcover editions, that make them arguably more enjoyable than when they were published decades ago.

If only DC would release Kirby's Fourth World books in nice hardcovers, then all his best work would be available in nice editions.

Although DC did just release Kirby's JIMMY OLSEN material in a new trade (JIMMY OLSEN issues 133-139,141). With hopefully a concluding volume of 142-148 to follow.
Kirby's JIMMY OLSEN is among my favorites of Kirby's work, it's tremendously fun reading. A great mix of action and humor. Too bad the Transilvane story (JO, issues 142-143) couldn't be reprinted for Halloween. Maybe the 2nd volume will be out before next Halloween.