Actually, I'd say that in "google" or "yahoo", "oo" sounds more like the German "u" rather than "ü". There's no "ü" sound in English really -- that I can think of.

The difference between "google" and "book" is more that "book" is a very short "oo" sound (so a German "u", but very short) whereas "google" and "yahoo" are long "oo" sounds. "door" is entirely different, in that it is pronounced more like the "o" in "Dortmund".

I'd say the general rule is that "oo" is a long sound, and "book" and "door" are exceptions to the rule.

Another exception is the word "good", which also has a short sound like in "put", "could" and "should".