Kevin Randle, the same fellow who co-wrote UFO Crash at Roswell and a few other tomes, edited this book, Faces of the Visitors, which shows
dozens of sketches of alien critters as described by their breathless witnesses. The only thing to say is, Either there are multitudes of alien races all "happening" to drop in on li'l old Earth, or they're all a bunch of danged shapeshifters, mebbe.
David Knight's book, UFOs: A Pictorial History from Antiquity to the Present, is a nifty compendium of sketches, photos, engravings (from newspaper reports of the 1897 Great airship Scare), and lots more.
Again we are left with the unbelievable conclusion that poor insignifcant Earth is on the receiving end of hundreds of differently shaped space vehicles.
Theories abound, from hogwash to complete acceptance of all comers. I myself tend to side with Jacques Vallee and John Keel, what's called "the Ultraterrestrial theory."