roulishollandais, you said books. Are we looking for several books, or several volumes belonging to a single title?
How many books were there?
FYI a world-wide library search for books in German written by L Prandtl can be found at:
Results for 'au:L Prandtl' > 'German' [WorldCat.org]
Do any of these titles trigger your memory?
Peter H
From the limited information so far -- and not having any German -- I'll just observe that Prandtl published a multi-volume work in German called
Ergebnisse der Aerodynamischen Versuchsanstalt zu Göttingen.
... which does seem to have the green covers you mention.
Text from [all?] four volumes can be found at:
http://tinyurl.com/kgk3c6j
http://tinyurl.com/ljws7un
http://tinyurl.com/kubffpc
http://tinyurl.com/lhl7v4r
... is this of any interest?