I will leave the choice to you as to whether you include it or not, I would just point out that if you make a statement like you first sentence, it's better to be specific, since it lends more credibility. Something like "according to seasoned ufologists Hynek, Valee, Keel and Elkins along with a large quantity of anecdotal evidence", something along those lines. That's something they drill into you when you study journalism, never be vague, be specific and identify your sources.
Good luck with your book, not a small undertaking!
Good luck with your book, not a small undertaking!