That's a pretty sweeping statement. Have you got any evidence that these books "helped form his ideas" or that he even knew they existed?
That Jack Kirby might have been familiar with some of the most famous and influential science fiction novels ever written? I don't have Jack Kirby's library checkout slips handy, but I'm banking on YES.
In MASTER OF ADVENTURE, Lupoff insists that one clear-cut inspiration for Burroughs was GULLIVER JONES ON MARS. If it wasn't for the fact that the similarities between it and PRINCESS OF MARS are so very shocking, I would discount this observation out of hand. Why? Because GULLIVER JONES ON MARS was a such a minor, minor book that Time Forgot. LORD OF LIGHT, on the other hand, is a seminal SF classic, Jack Williamson is the guy that coined the term "genetic engineering," and (this is surely the topic for a whole other thread) A.E. van Vogt may be, along with Don Rickles, one of the largest, most uncredited influences on pop culture.
Like DUNE or PJF's RIVERWORLD, LORD OF LIGHT is something that is so ubiquitous that the burden of proof should be on showing Kirby DIDN'T read it.
I'm not saying that Jack Kirby is a plagiarist; if LORD OF LIGHT was the starting point for the Fourth World, Jack's creation became something else. What I am saying is that the Fourth World would not exist - IN THE FORM THAT IT ACTUALLY TOOK - without Zelazny, his imitators, and the entire "space god" theme in science fiction starting with A.E. van Vogt in 1940, among others. Because of many similarities to LORD OF LIGHT, it's interesting to place Kirby in that science fiction tradition.
There's a tendency with some creators to not analyze their works as being products of a larger culture. Kirby isn't alone here; Woody Allen's movies have such an idiosyncratic personal vision that usually, they're only compared to other Woody Allen movies. The Flaming Lips created a subgenre that pretty much only consists of themselves.
People don't find it unnacceptable to look at the science fiction influence on other characters. Nobody bats an eye when the geneology of the Green Lanterns can be traced back to E.E. Smith's Lensmen, or that Ray Palmer would not exist in his current form without THE GIRL IN THE GOLD ATOM. In a general sense, we talk about how Frank Miller was shaped by Eric Van Lustbader and Spillane. I say, why not do the same for Kirby?
I just received my Eternals omnibus yesterday. Inside the back cover flap is a brief bio of Kirby. Most every book does something like this, and usually titles it with the author's name, or the word "author" or "artist," etc. But written in big bold letters across the top of this bio blurb, it reads: "THE CREATOR"
Amusingly, ETERNALS is the best example I can possibly think of to make my general point: Kirby is shaped by outside influences and the culture around him
just like any other writer or creator. People say Kirby and his stories are "timeless," but ETERNALS could not have been made at any other period, in any other zeitgeist, other than the mid-to-late seventies. This was the era of the UFO generation and CHARIOTS OF THE GODS?, the Golden Age for schlockumentaries narrated by Leonard Nimoy, Orson Welles and Rod Serling that saw innocent cave drawings as signs of extraterrestrial visits. Where, in THE OUTER SPACE CONNECTION Rod Serling, with a totally straight face, called the perfectly normal ruins of Tiahuanaco "Earth Base One."
People say CHARIOTS OF THE GODS? was THE DA VINCI CODE of its generation, but that would actually be underestimating it. What CHARIOTS OF THE GODS? didn't pull in terms of raw numbers, it was a flashpoint at which dozens of factors converged: the phobia of UFOs, the burgeoning advances in space travel, the beginnings of historical revisionism, and the "New Age" movement which combined pseudoscience with religion.