I'm not sure I can take any UFO books seriously, anymore. The most honest treatment of any UFO sighting would end in the phrase: "...or it could've been, you know, whatever."
Frankly, people put too much credence on their own observations. The human sensory system is amazing by any standard; really, a marvel of biological engineering and evolutionary achievement. But, c'mon, we're routinely fooled by optical illusions, tricked peripheral aberrations, and stumped by lateral inhibition.
I've seen UFOs. For real. But I certainly don't ascribe them to alien intelligences (as many do)... I simply disregard them as "you know, whatever."
That being said, maybe this book doesn't handle UFOs in the traditional manner. Kaku seems to have something nice to say; but then again, he's been getting more and more fanciful the whiter his hair gets.