The Space Egg, by Russ Winterbotham

In a remote part of Kansas, test pilot Jack Fayburn tests the experimental XDW-49 for Darling Aircraft Corporation. Somewhere at the edge of space he encounters--something. When he returns he is not the same man. He's withdrawn, aggressive, he possesses super-strength and a super-human healing ability. Bullets don't stop him. Jack's condition spreads: Ruby Cascade, C.F. Darling's secretary, starts exhibiting the same behavior.

In no time at all, Jack and Ruby seize control of the facility (the Darling test site is remote and isolated) and cut the communication lines. Jack and Ruby make it clear they are a new sort of creature, and soon everyone on Earth will become like them—or die. Photographer Bob Reeve, Dr. Felix Maynard, C. F. Darling, the sheriff, and the rest of the staff try to stop them. It's hard though: they have superhuman healing powers and can absorb bullets like so many Nerf darts.

In the end, Bob appeals to Jack's humanity. He calls out to the shred of the original human Jack who is still in there, and Jack responds. He grabs Ruby and they self-immolate in black flame. Earth is saved, the end.

It's not a great book, but I always enjoy a story set in Kansas.