Comprehensive histories of Ufology may be found through the following resources
Comprehensive UFO History Books (richarddolanpress.com)
Topical Ufology Guide (ufoevidence dot org was recently hacked by scammers, this is an archived backup)
Other Helpful Links in Researching
Chronology of Significant Space Science & UFO Developments (adamskifoundation.com)
Large Library of UFO (and other) PDF Books (avalonlibrary.net)