Why have sightings of unidentified flying objects around the nation more than tripled since 2001? Why did they spike in Texas in 2008, or in New Mexico in September 2015? These questions and many others emerge from the first comprehensive statistical summary of so-called close encounters: 121,036 eyewitness accounts, organized county by county in each state and the District of Columbia, from 2001 to 2015. The unlikely compendium, U.F.O. Sightings Desk Reference," is the work of a couple in Syracuse, who crunched unruly data on U.F.O. reports collected by two volunteer organizations: the Mutual U.F.O. Network, or Mufon, and the National U.F.O. Reporting Center, or Nuforc. It is the reference "U.F.O. researchers dreamed of having," Gordon G. Spear, emeritus professor of physics and astronomy at Sonoma State University ... writes in the foreword. Rebutting a common perception that U.F.O. sightings are on the wane, the Costas' book shows that sightings have risen in waves, to 11,868 nationwide in 2015 from 3,479 in 2001. Only a small fraction of sightings are actually reported to Mufon or Nuforc. The Costas refrain from speculating on what exactly is happening. For the U.F.O. enthusiast, the pages of graphs and charts are a treasure trove of hard-to-find detail. The arduous breakdown by the nation's more than 3,000 counties was notable for revealing clusters of sightings in ... places where U.F.O.s are almost never mentioned. But every county in the United States appears to have seen at least one U.F.O.