I can't believe nobody has picked up on this; it's really pretty easy to identify; I would have identified for you earlier if I had seen your request. (Fact is, I've been selfishly too concerned to see if anybody can help me identify the book I badly, badly hope to have identified: but nobody seems to have a clue!) Anyway, rather than to give a long answer, I'll give you a short one, but one that will give you all you need or even want to know to put you on the right track. The author is W. A. Harbinson, the work as a whole is entitled Projekt Saucer, and runs to five volumes: the various copyright dates on volume 1 (to speak of just vol. 1) are 1991, but also 1988, and even 1980). All five were published in the UK, but only two--vol. 1 and vol. 3 (yes, vol. 3)--were published in the US, where they were still called Projekt Saucer, but identified as Book One and Book Two.
Like you, I have fond memories of this work, though I knew it in the 1990's only in its abbreviated American form. Since then, I have secured the five-volume edition in paperback from a Canadian book-dealer; a bit pricey at $100 US, but I was happy to find it in VG condition, so have no complaint; the two volume US abridgement I was able to secure here in the US for a tenth of the cost of the set from Canada. My point in talking about prices is merely to give you an idea of what to expect if you decide to go a-searching; even finding a complete set of five volumes proved to be a bit of a challenge.
Additionally, Harbison published (copyright 1988, 1995, and 2007) a single volume historical account of UFOs, in which he provides the historical basis for a large part of the five-volume science fiction novel. It is titled Projekt UFO: The Case For Man-Made Flying Saucers. I bought my copy about a year ago, from (as I recall) Amazon.