Hi Mike,
Welcome to the Forum!! I hope you return frequently, and take part. The forumites who frequent this place include experts on just about any subject you could ask about in relation to WWI aviation.
Of the three books you mention, my own preference would be "September Evening". It's well-researched and an entertaining read about
Werner Voss, though I don't agree with everything the author says.
As for the other two, well..."German Knights of the Air" is a decent introduction to the German airmen who won the Pour le Merite, but it's pretty dry and not particularly well-researched. I could say the same about "Knights of the Black Cross", which, if it's the book I'm thinking of, is by the same two authors and rehashes much of the same material.
If I may be so bold, perhaps you should check out Osprey Publishing's series of titles on WWI Aces at
www.ospreypublishing.com. Go to that site and search around and you'll find many titles. These books are all in print and can be found at many, many outlets such as Amazon.com or Barnes and Noble. They are affordable, well-illustrated and give a very good introduction into many different personalities and their aircraft. Any titles by Norman Franks or Jon Guttman are well worth obtaining.
If you're a von Richthofen enthusiast, any of the books by Peter Kilduff on the subject are extremely well-written and highly researched. If you should be interested in Allied pilots to provide a balanced perspective, I would suggest Charles Woolley's two books "First to the Front" (the 95th Aero Sqn) and
The Hat in the Ring Gang (the 94th Aero Sqn). Both are not cheap but extremely well-illustrated and full of very deep and fascinating info. Both are available from Schiffer Publishing (or again, other outlets). As long as we're talking Schiffer, you can do no better than to pick up Mike Westrop's "A History of No. 10 Squadron, Royal Naval Air Service" for an in-depth study of one of Britain's most famous fighter squadrons.
I'm sure other forumites will have many other suggestions. If you do a search through the forum archives, you can find several threads on "the best books" in our field. Or check out the book review section on this very site.
Best wishes,
Greg