I've read it and also found it interesting but unconvincing. It focuses almost entirely on American sightings in relation to their defence networks and the US government. By the end the author seems to be of the opinion that the government knows a lot more about these things than it's prepared to reveal (the usual stuff; Area 51, Roswell, aircraft radar anomalies, etc.).
It's odd that with everyone carrying a decent camera with them these days, the best we can do is fuzzy indeterminate blips on aircraft radar.
There are some extraordinary claims kicking around various media, the oddest suggesting that aliens have been living beneath the surface of the Earth for many millennia, and are in regular communication with world governments.
The truth, I suspect, will turn out to be far simpler.