story identification - Looking for sci-fi book about alien invasion where Earth builds a giant laser to stop an incoming alien fleet

Looking for a science fiction novel I read as a kid where a few hostile alien saucers show up and the US Air Force is trying to bring them down. It's too fast for any missiles/cannons to work, and finally the only way they hit one of them is with some sort of experimental laser carried inside a cargo plane. Later on, they detect a large alien fleet far outside the solar system, heading for Earth. Assuming this is an invasion fleet, they embark on a crash course to construct a gigantic laser (I believe it was gamma-ray based) to fire at the alien fleet before they get close enough to Earth to spread out.

I remember some sort of protester/activist tries to fly a small plane right down the barrel of the laser just as they are about to fire it and gets vaporized by the beam. The laser works and spreads out enough to hit the entire fleet at once. The radio chatter from the fleet increases after the laser shot, then gradually dies out to silence. The fleet of alien ships (presumably full of dead aliens) drifts aimlessly past Earth out into space. The book ends with one of the chief scientists/generals explaining to a bunch of world leaders how this is only the first step, how the aliens are likely to return, and how they need to expand, jump out into the solar system, build more giant lasers for defence on all the planets and moons. The world leaders are silent with horror, and the scene ends with everyone staring at a ticking clock on the wall.

Answer

The novel is Invader by Albert Fay Hill and David Campbell Hill. The story matches your description at every point: alien saucers, interceptors destroyed, huge gamma-ray lasers (called Grasers), alien craft drifting by after being hit by the laser, and the need to lay out huge sums to defend against the expected next invasion. The book ends:

"In general, here's what we suggest: even more powerful Grasers strategically placed-- first, all over the world, then on the moon, on Mars, possibly in some of Jupiter's moons. And when we've developed our technology enough, we must build mammoth satellites, armed with the most advanced weapons, to orbit millions of miles from earth. Then... "

As he talked, Beck's enthusiasm became more ardent and his voice rose to its usual booming volume. But suddenly he sensed that the silence was too complete, that despair around him was as deep as the pit of hell. He looked up from the notes he had been reading. And he saw the leaders of eight nations staring at him in mute horror.

The only sound was the relentless ticking of the clock.