"This might well indicate that we have been visited by not one lot of aliens, but several different lots of them. Why should there be only one lot of aliens capable of interstellar travel? Would there be any particular reason for only one? I can't see why. If there are any, there may be many. Some may be humanoid in appearance, and some not."

LH, this is similar to the point that I made in my post of 16 May 08, ie.

"In a galaxy containing around 100 billion stars there are likely to be more than two (us and the UFO pilots) intelligent, technologically sophisticated species and all possible body plans (whatever they are) could be represented."

A little clumsily expressed, I admit, but a very similar point all the same.

As for reading about UFOs - most of the books that I have read on the subject seem to have been written by fairly obvious charlatans - can you recommend some books that you consider to be credible?